@OP, I think it's a really nice suggestion to add chess as a gambling game. Chess is a form of recreational activity that often played in person, but now can be played online so it can really fit into the games on the gambling sites.
It is good to reconsider because like all other gambling games, this requires critical thinking and different strategies to win. This will be a great opportunity to introduce traditional sports to gambling community as well. And if ever it'll be considered anytime in the future, no doubt more people will be hooked and play this for entertainment.
Believe me or not but there are only small percent of gamblers that are interested in chess than other physical sports. I think also that Lebron James is popular than Garry Kasparov.
Casino`s really love bettors and they are after the bets than in sports. If only few gamblers have interest in chess sports then there is only small amount of wagers every time there is a tournament. I don`t say that chess isn`t popular, it is just only small percent have interest to bet. But then, who knows? It may really give an impact to gambling world since a new sport may be added.
And about this, I think I have to disagree with you. The reason why you think there are only few gamblers who might be interested in chess in gambling is because you normalized poker, dice, and other games in gambling as your main thing.
Try to get out of your comfort zone and visit the sites that OP had mentioned. You'll see what I'm saying. And I am very sure that if they include chess on sports betting, I bet that would be hype as hell. They just need to allocated a fix time for every rounds so it won't be boring.