well, clearly not many people want to play 64 player games. i mean, yes a few have been popular, but the length of time for which it's easy to find plenty of busy servers that aren't too geographically removed from you is tiny unless your game is a huge hit. COD could probably do it with name brand alone. BF3 likely COULD have done it but chose not to go after that quite as hard as before.
for me, once you start getting more than say, around 24 players in a game, what i don't like is the loss of relationship. a game like MAG with over a hundred players, there's no way i can keep track of that many players. when you play with fewer players, you develop a narrative with them almost. you remember what you did to them, and they did to you, and you react accordingly.
when a game has more individuals than i can really keep track of, it loses something for me. i much prefer being able to get a sense of the play styles and personalities of the people i'm playing with and against.
that isn't to say i didn't have loads of fun with Wolf MP and Wolf ET, because i did, but i had more fun with them when there was about ten or twelve players on each side.