One problem is, once they're full, there's no queue system to wait in line. So you go to try find an empty server to start populating... and then get stuck on the lobby... for 10 minutes.
There's nothin more boring than sitting on a blank screen with the same music rolling over. I say scrap the lobby screen, just load up the level and just have it as free-for-all until the min number of players are in, then restart the stage. People are far more inclined to stick around a low player number server if there's something to do.
tl;dr The lobby waiting system is killing the MP. Let people muck around in the server until a min number of players join.