This is a video I found to better explain why, especially for anyone (though I cannot imagine how) who is sitting on the fence about how bad dog fighting is, or those apathetic about it. It is DEFINITELY not for the squeamish, but I reccomend watching it - lots of interview footage with Humane Society officials interspersed with video footage of some of the most ghastly things I have ever seen. It made me sick to my stomach and I started to cry when I saw the dog with the broken legs near the end that they were still trying to fight. Be warned.
Years ago, we adopted a pit bull who was on the brink of death - she'd been abandoned and tied up in a shed with no water or food for days and had a serious (life threatning) skin condition that the vet was amazed she survived. We brought her home, nursed her to health, and she was the sweetest thing you could have imagined - but she never got over her aggressive tendencies and in the end we had to give her away to people who had no other animals. She kept fighting our other dogs unprovoked, and it was too dangerous to have her at home with our other dogs as well as Cole, who was only in Pre-K at the time. Elly was a sweetheart, though. I miss her. It is in part because of Elly that I feel as strongly as I do on this issue, because I've seen first hand what these people do to the dogs they raise and what conditions they leave them in as a result.
I would also like to mention that Maddy, the other pit bull we got from a trailer park family that was fighting her, is still going strong and while she is mostly nonaggressive (95% of the time), it's been an uphill battle. The only thing she really hates and wants to fight now is motorcycles - especially Harleys. She's eight years old, only has one ear, eight teeth, and is narcoleptic - but we love her.