Because halo is an overrated game. I never hear no CoD4, counter-strike, bioshock, HL2, etc. in my school.
At least in GTA you do not hear a 6 year old singing a terrible remake of Umbrella, and it has freedom. Halo to me isn't that great, in halo 3 they added a few things, which made multiplayer stuffty.
I got so bored of shooting enemies in halo....