One correction: cockfighting was not banned in all 50 states a century ago. It was legal in OK, LA and NM until a few years ago (and it still is legal in PR, a U.S. territory). The federal law in question covers the transportation across state lines of the animals and equipment used in cockfighting.
It might have technically been “legal” in three states a few years ago, but its similar to the beastality laws I was arguing with some lefty gay marriage advocate about. He was claiming zoophiles had it better than gays because it was “perfectly legal” to have sex with a horse in a dozen states or something due to lack of bestiality laws there. While it was true there was no specific law in those states saying “no sex with a horse”, anyone who was caught doing so publicly could still be arrested and charged with other crimes (inhumane treatment, gross indecency, etc.) due to animal cruelty laws already on the books in those states. A few years back, some sicko in Florida had sex with a dog where its technically “legal” there, and he ended up in prison.
The same would likely be true if anyone had tried publicly hosting a public cockfighting match and having people bet money on it in the states where it was still “legal” due to a loophole. If they were caught, there would likely be a public outcry and they’d be charged with a related crime and be punished for their actions.
I’d also speculate that the most likely reason it was still legal until 2007 or so in those specific places was that state legislators simply weren’t aware that there was no specific state statute in effect banning it. Most first world countries banned blood sports like cockfighting, dogfighting, etc., by the time of the civil war.