Reason One: They never end.
Set a goal for the game. End the game when it reaches this goal. This can be anything from "Get 100 players in a game" to "Kill the Prince" to "Until it's too cold outside to play" -- just choose an ending. If you don't, the game will die -- of something. Better to end it on a good note than watch it suffer and die over a long period of time...
Games must have goals. Too many LARPS don't.
Reason Two (and the biggest reason): Different player goals.
Player A wants to write up his kick-ass Brujah and go to town on some creeps. Conflict (and victory) is the primary reinforcer for the player -- be it through combat (kicking ass) or socio-political shenanigans.
Player B wants to "live life as a vampire in the World of Darkness." This involves "inhabiting the skin" of the character and becoming immersed in the game as if it were real.
Player C wants to create a story NOW (as opposed to recounting events in a breathless, "...and then..." manner) about their character's current circumstance. This player is more apt to "arrange" scenes in advance, use out-of-character info (in a positive way) and ultimately, deal with the circumstance and stop playing once the character is "finished."
Now, you can disparage any one of these styles. Whatever. Doesn't matter. The point is that they don't work well together. Player A & B see Player C's use of OOC info as "cheating." Player B is a victim of "Well, that's what my character would do!" (often to the detriment of the game, other players' enjoyment, etc.). Player A is labelled a powergaming munchkin and is seen as "disruptive" by Players B & C. Player C wants to be able to play his/her character without fear (of embarrassment, death, social exile, etc.).
Lastly, the MET system says it does one thing (promotes "storytelling") then weighs down the system with lots of options (mostly for combat) in the name of realism and encourages munchkinism by setting up a situation where a) the Most Powerful characters have the most to do (and therefore the most fun) and b) making the system reward Killing Other Players (diablerie, natch).
That's why Vampire LARPS fail.
I've been doing this since Night Owl started doing WW LARPS back in the early '90's (which makes me a crotchety old gamer-man). I've played in 'em, I've run 'em...and yeah, I've been frusterated beyond belief by
The solution? Well, that's another post for another time.
(oh yeah, Fuck Timestops)