Because there's absolutely no chance for randomness; the bullet is gonna go where you shoot. This makes shooting a pure test of who shoots better. With bloom, the reticule expands (obviously), giving you a better chance to hit someone even if your aiming is off. And also, Halo plays 10x better without bloom...But bloom isn't all as bad as people put it out to be. There is a little skill involved with it, but Reach couldn't pull it off.