My uneducated guess is that it has to do with the people who tend to buy pits in this day and age. I remember I used to see them in music videos all the time (I don't know if this is still the case) and that mixed with people doing backyard breeding makes a pit not very expensive. So people buy a cheap pit, don't train him because they can't afford it/don't care, so you have a breed that is bred for defense but without training and it will lunge at anything that comes near its territory. Its not like pit bulls are the only aggressive dog breed, there are others but they tend not to be as cheap, rots and dobermans aren't very cheap so if someone buys one, generally they will invest in some kind of training. And I'm not even touching the unfortunate area of dog fighting, another area where pits are extremely common.