I suspect the reasons are (generously) keeping them out of the hands of people who would treat them poorly and perpetuate the stereotypes, or (less generously) ignorance and fear.
There are plenty of statistical studies out there that pit bulls specifically cause both a significant plurality of dog bites and significantly worse injuries than other dog breeds.
The word "pit" in "pit bull" refers specifically to a dog fighting arena where dogs are supposed to aggressively fight each other in a duel, possibly until one of them dies.
"pit bull" refers to a dog breed that was optimized for its performance (=more aggressive and dangerous) in the "pit".
How they're raised makes a big difference, but natural instinct is natural instinct. It's just like how chihuahuas were bred to be small, but pit bulls were bred to fight other dogs.
