One thing that hasn't been mentioned and I find VERY relevant is the fact that the Main Events at an anime con is always being used for other things and the Masquerade takes second place during the day. Very few conventions will schedule more than 2 hours for rehearsal, because they NEED that room for the AMV contest, a movie premiere, concerts, etc.
That makes a big difference on how the staff has to handle the event and how much attention can be put on individual entries. Anime cons just don't have enough time, which would make a huge difference on keeping people relaxed and well-practiced. It would also give the Tech crew a smaller margin of error, which would be nice...
Also, the green room issue. It's a panel room that at larger cons, will more than likely be coveted for actual panels and workshops. A lot of conventions don't allot the use of the large panel rooms for the cosplayers because they've got other programming for the con scheduled. Sometimes, there isn't a room to even spare, such as times you're stuck in a line going through hotel staff hallways or loading dock areas.
So one huge factor overall is that the convention itself restricts what the Masquerades can provide for their competitors' comfort. Conventions that pride themselves on good costume contests will make ample provisions for the costumers. Conventions that see the contests as just a major event to pack in the Main Events won't do that.