Because they are dangerous, unpredictable in behavior, and unnecessary. I've heard all the reasons and rationalizations posted here. They don't hold up if you examine them. I have a friend who owns several dogs including a pit. When kids play near the fence that contains him he goes crazy trying to get at them. And yet he will come to me and lick my hand. But I still don't trust him.
I have another friend who owns rent houses. She can't rent to anyone with a pit. Her insurance company will just cancel her policy. That's pretty telling evidence that owning one of these dogs is very risky..
Beyond the Interview: Father of Child Killed by Babysitter's Pit Bulls Speaks Out After Attack - DogsBite.org
I could go on... and on. I could post pics of some of the children mauled and killed by pit bulls and links to the reports and news articles but I'm sure the mods would delete them because they are graphic and disturbing. There are so many myths. Dog-aggressive? No. That's a phrase used to defend and confuse.
If only qualified and responsible people owned them there would be very little risk. The problem is, anyone can get one. To me the risks aren't worth the life of even one child.