Well, the popularity of football cant be denied. Thats why, because of that, there are a lot of audiences, and because of the hype of the people about a certain team in football or player, it adds up to the more popularity of football, so we can also expect that in sports betting, football is the leading sport that people bet on. Well, for me, I don't usually place bets on football because in my country, football is not so popular. Maybe because football is in the western country and we know the population of the western country is great, it's easier to say that it's because of the population. I love the NBA, and thats where I bet all the time. Even if there are not many promotions or events on the NBA's casino platform, it's fine as long as I can bet whenever I want and when I want to.
Popularity is I believe the major factor on this aspect. Though basketball comes second to football, maybe the sponsors are investing more on football. And that it may also depends on where you are living at. If your country is more on basketball, I believe, the local sponsorship will be on basketball. But in general, we can see based from statistics that football is more popular than basketball.
Also, bookies have their own stats as well, so maybe in their internal numbers, they have a lot of bettors on football over basketball. Who knows? But if they are giving some promos, it means they have very good reason to do so, right?