It's the breeding since about 1989 or so.
Proprerly bred Pit's is the problem.
If you read the history of the PitBull I think you will find that in the past any pitbull that even offered to bite a human was put down. They were considered too dangerous.
Then along comes the badboys and gangbangers wanting there pose dog and some breeders even breed em to bite people now.
Training em to NOT bite people can be tuff. So a little note that MAY help if you buy a pit.
Select your puppy. Place your hand near the puppies mouth. If the puppy puts It's mouth on your hand or trys to chew on your hand then recommend you find another puppy. Possibly even from another litter.
The puppies can be very different even from the same litter. One will actually try to chew on your finger where another will turn it's head away and avoid it. The better bred ones will require you to force your finger into their mouths.
Now comes the fun part! Training.
If they even offer to bite in play

.....
just get another dog

That is MHO.