Because they are sharp and hurt like a ***** if you grab them off of spins?
Because it's just one more thing to break?
Because if you're smart you don't need to lock your grips to get them not to throttle?
Because they're twice the price and last half as long?
Plus they're usually narrower and thats a pain in the ass as well, they're good for MTB's if you change bars or hand controls a lot or if you don't move your hands a lot. In BMX they're gay.
__________________
"the bus came by and I got on, that's when it all began...there was Cowboy Neal at the wheel of a bus to never-ever land."