Because Bungie dosen't like that stuff the truth is that there are also kids who play Halo and you know they can be scared of the Flood but blowing of body parts will scare them even more and halo will feel like dead space.Too much gore = more wakos playing
It is propably to make the game somewhat safer to younger players. There is a little gore: when You kill the Flood they usually explode.
-- To the above answerer, the game is rated M, as in 17+. You are SUPPOSED to be 17 years or older to play it, so the younger players should not have an opinion in the matter. On the other side, it is nice to play a game that doesn't involve you dismembering an enemy or killing them in a horribly violent way.
--actually in halo (possably H2,and H3)you can turn of all the blood and stuff its very help full...