Because the media reports any and all pit bull episodes. Also, this is the breed that is associated with underground dogfights.
As with most pet issues, you can start by blaming the handlers. If an animal is not cared for, trained and handled correctly, it will become a problem.
I was recently chased by 3 pomeranians. I had gone to look at an item a lady had for sale and she said to just come in her yard and park, which I did. When I got halfway to the boat, these 3 came growling and racing off the porch. I ran to my car and literally dived in. They retreated and went in the house. I tried a second time, same results. Finally the lady came out. She had been unaware they'd gotten out, because someone hadn't latched a door securely. I have no doubt I would have been bitten had they got me, because they were protecting "their" premises. The only 2 dog bites I've ever had were from Jack Russell terriers. Once in awhile, the media will report other breeds that cause trouble, but they seem to love to target the pits.
I have 2. One is a half pit/boxer and he followed me out of a store that his foster mother owned. I felt it was destiny I should get him. The second one was found wandering on a street, she is full blood pit and was bred at an early age, by the looks of things. Both my dogs were immediately spayed/neutered. I could not stand the thought of baby pits ending up in the wrong hands.
I don't know if my dogs would ever "turn" on me, or anyone else. But I consider myself a responsible owner. I know that people have a natural fear of them, so they are always leashed and are not allowed to walk up to strangers. If someone wants to pat them, I let them, but they must come to me first, we don't push ourselves on people. My dogs are never in the yard alone. Too easy for them to dig out or jump -- my big dog jumped a 5 foot fence with ease one day. Of course they are not being trained to fight or do bad things, but they are naturally protective of my house. I live alone, so I am glad for their bark.
I couldn't have asked for better companions; I love them and someday I'll have to say goodbye which will kill me. But they are not for everyone, and people who own them, must be responsible. And I'd like to see them removed from the hands of what I consider "bad" owners; not be euthanized, but to be saved if at all possible.
Detroit has a law I saw on Animal Planet, that when they confiscate pits, they automatically euthanize them. I think that this horrible. And yes, it included puppies and even the people who brought the pups in were crying, in an episode I saw. Many towns have banned them altogether.
All dogs deserve a chance, and should be evaluted individually.