Growing up we had a pit bull, very nice dog as long as the dog new that person. That being said that pit bull was very viscous towards other dogs, you could not take that pit bull around other dogs, it would immediately try to attack any other animal. Looking back at it now we were lucky we never had an incident. There is no way in hell I would ever try to raise a pit bull in my house. Just not worth the risk.
I think it is a combination of the type of people that own pit bull and the dogs traits that are built into their DNA that make for a bad scenario. Not all the owners are trash but there is a much higher percentage of trashy people the do own pit bulls.