The difference between a small-scale dogfight and a real championship dogfight in 1957 is the audience. At the former, the all-male crowd is stereotypically country-bumpkin with faces and clothes covered in dirt. At the latter, rich folks are present, including women wearing their pearls and Sunday best. The surprise isn't that each has their own demographic, though; it is that either has any enthusiasts at all. Perhaps it is the illegality that draws them in, or maybe it's that gamblers will bet on just about anything.
While I was thinking about how dog fighting would be a tough-sell for a film like Walker Payne, which stars Jason Patric as a novice of the sport, I overheard some people in the audience discussing the contrary. They claimed the picture would be more marketable if the filmmakers cut out the dramatic story and just kept the dog fighting. If there are in fact people who enjoy watching pit bulls kill each other in a ring, then that edit would certainly make sense, since the film's narrative has very little going for it.