So why does this happen? Well simply put, MH carries within it the seeds of its own downfall. The game's creator spent a lot of time making a game where there were many opportunities to RP, and many different scenarios could be formed that would provide enjoyable gameplay to all. He also, unfortunately, undid all of his hard work by creating a bunch of pointless, game-breaking weapons and hiding them in specific locations that you could only find if you knew where they were OOC-ly. This in itself is an open invitation to all would-be griefers (see SS13), and more or less provides the groundwork for a disappointing RP experience to come.
"Space Station 13" syndrome (named after the game where this type of behavior runs rampant), is what usually happens very quickly after the game starts. Instead of bothering to play properly, experienced players with a chip on their shoulder (read: noobs) will go off, seek out all the best items on the map, arm themselves to the teeth with said weapons, and then roam the hallways in gangs or alone, bashing everyone they meet to death. It's affectionately termed "Random Killing" in the world of MH, and understandably, upsets most of the good RPers. What's worse is that due to the miraculous inability of most hosts to actually do their job and ban these people, it creates an artifical power hierarchy, where the killers become idolized for their power and others will flock to their side so as to prevent being killed over and over again. Without a stong system of authority, these players will rule with an iron fist, killing and sparing whoever they please with nobody to stop them. In an ironic twist, it's exactly how MH was meant to be played, minus the complete lack of roleplay.
Now, I could go on for pages about the hosts not doing their jobs, but this is a review about the game, not about the morons who play it. Game-breaking weapon exploitation aside, there are other areas where MH lets itself down into an even greater spiral of incompetence.
First and foremost of these is that the game essentially sets itself up to be a random brawler rather than an RP. Yes, it's a survival game, but when half the students in the school apparently keep knives in their desks and it's a common occurrence to find meat cleavers and syringes in the bins, you know that death and stabbing is just around the corner. It's also important to note that there are only a few items NOT related to combat, and these items are largely useless (such as the flashlight or the ketchup bottle).
Secondly, the game doesn't clearly define the limits of people's knowledge with regards to the RP, and some take this as an excuse to more or less know "everything". Players in "Death Note" mode will demand to search everyone in sight with no valid reason (and of course will freak if they find a death note), or start stabbing or be suspicious of anyone who they catch writing, regardless of the fact that they don't have any reason to worry RP-wise. The truth of the matter is that in DN mode, Kira should have pretty much no trouble killing everyone, since there's no valid RP reason why students would withhold their names from each other. I illustrated that in a comic for this game, as seen below:

Of course, the reality of this is much different. Nobody wants to be a victim, especially when dying means you float around as a ghost for up to an hour, watching the living people have fun while all you can do is chat. It's boring, after all, so people essentially cheat and make up excuses to make Kira's job almost impossible. What this means is that the death note is utterly useless in anyone's hands except for the "shinigami eyes" player, who can see everyone's names. However, that player and Kira rarely meet, because there's no easy way of distinguishing who's who without making everyone suspicious. So in the end, it's only a matter of time before the RP ends and the RK starts.
Thirdly, the entire RP element suffers from enormous plot holes. Let me demonstrate by pointing out the premise here: This is supposedly a normal high school, yet only one teacher exists on campus. One. This teacher is apparently a superhero, because she must somehow teach in like 8 different classrooms simultaneously. When she dies (kryptonite?), the super advanced computerized system of Mitadake High locks the school down and presumably cuts the phone lines too, since checking the phones in the game will always result in the same message ("It's dead."). Clearly, its more imporant to prevent the killer from ordering pizza than it is to prevent him from killing off the rest of the students.
Apparently not satisfied with this level of plot devastation, the plot then goes on to point out that students all carry around high-tech PDAs which have a range of useful functions, but nobody bothered to bring a cellphone to school with which they can call the police. That's ok, at least everyone brought glow sticks, smoke bombs, syringes and knives! Wait, what?
Not only this, but the school's total lockdown can be overridden with a security card (found randomly inside student's desks for no apparent reason), but it won't let you leave unless you take the card with you, meaning only two students in the entire school can leave (until 24 hours have passed, in which case the system changes its mind and lets everyone out, thereby completely ruining the point of locking the school down in the first place). The cards are apparently issued on an honor system basis, where the killer's own guilt will prevent him from using one of those cards to leave the screaming, angsty bloodbath behind him long before "help" can arrive. Oh yeah, and nobody is allowed to use the vending machines after 4PM. So take that, killer!
The only way ANY of that makes sense is if all of the students are secretly part of some top secret scientific experiment and the sophisticated Mitadake High AI is actually the real killer. Since it's highly implausible (despite video 'evidence') that a student would kill the only teacher, get rid of the murder weapon and then teleport back to class before anyone actually notices he was gone, it can only be explained that all of the "students" (test subjects) are brainwashed by neurotoxins fed to them via the vending machines (before 4PM obviously). The only thing missing from this theory is (aside from a portal gun) is a creepy, mechanical female voice that announces random things while students wander the halls. OH WAIT, THERE IS ONE.
As a final slap to the face, the big security doors that lead to "freedom" probably just lead to a firing squad to pick of any survivors who make it out alive so that nobody can ever live to tell of the bloodbath that was Mitadake High.
Of course, Death Note mode totally ruins that theory, so we'll just assume that Kira is in on it and he's given the note to a "student" just for laughs while he secretly watches them all from the AI's master control room.
I could go on, but I think you get the point about the plot by now. Long story short, the plot makes no sense, and the players' unwillingess to get slaughtered for the sake of RP often leads to random dashes for the bat room or people slinking off to find an alondite so they can go wave their phallic symbol around and pretend that slaughtering other players' characters can fill the void in their life where social skills should be.
Getting back the the review here, the last feature that fails the game is the custom map maker. While in the right hands this could be an excellent tool for extending the RP, more often than not it simply leads to arena-based maps where the RP is totally stripped away and all that's left is the killing-each-other part. Either that or maps where people think it's cool to put like 40 alondites and 8 death notes around the place and see how long it takes everyone to kill each other.
So in the end, this game starts up with a great premise but then trips over its own oversized shoes and falls on its face. With proper rules, better maps, and no freaking super weapons, the game CAN provide decent RP grounds, but the list of requirements for this to happen is very strict and the people who can pull that off are few and far between. In the end, Mitadake High is a lukewarm RP experience at best, and a random free-for-all brawl at worst. If you're looking for a good RP, keep looking, 'cause this isn't it. However, if you're looking for a game that undulates wildly between RP and slaughter, then you might just have some fun.
Edit: Cavern has pointed out to me that I didn't really touch on the good points of the game (which there clearly must be since I didn't give everything a 0). It's a valid point, so let me quickly paraphrase what I DID enjoy about Mitadake High.
Firstly, the graphics are very nice and the interface is quite well-designed. This makes picking up the game quite easy, even for newer players, though mastery of those controls is a separate matter. Attacking is a simple matter of equipping an item and clicking on someone next to you, and most of the controls are again a simple click interface. My main complaint is that there's about 3 different tabs which the controls are separated into, making it hard to do more than one specific thing at a time (ie you can't keep an eye on your energy while reading your PDA messages. That sort of thing).
Secondly, RP, while rare, does happen, and it happens more often than on say... SS13. When you get in a good RP, a game can be almost total junk and it's an enjoyable experience simply because of the interaction with other literate players. However, since the game's many imbalances reduce RP opportunity a lot, I didn't want to stress the RP factor very much.
Thirdly, and probably the reason I keep coming back, is the immersion. The custom-made maps often lack the realism of the default one, but the default map itself has a pretty much fully-furnished school, even if most of it is kind of pointless. The upside to this of course is that it provides a rich environment for RP fans to flesh out their own little stories.
And lastly, though broken, sticky, and often frustrating, the combat in the game can occasionally improve the RP experience too, particularly if the good guys win. When you manage to just barely defeat the killer in an axe fight to the death, or narrowly escape the maniacal knife-wielding bozo, you get the added bonus of an adrenaline rush and the ability to limp into the room covered in blood and have fun RPing your injuries.
So yeah. Though not nearly enough to redeem the game from it's frustrating spiral of annoyance, Mitadake High certainly has good points, and if EVERYONE RPed correctly then it would be a fantastic experience. Of course, getting everyone to play along is the tough part...