The fact that it is a casino doesn't necessarily mean that they have to use ".casino" As domain. Using that domain does not make the casino to be special in any way and because they have always prefer the old choice (.com) which is very easy for people, I feel that's why they continue to use it.
Actually, this makes them special because I never saw a casino that has this domain extension yet. It is not the domain extension is what makes easy for people I believe but it is the domain name. I mean even if they have that dot com extension but their site name goes like> 'cvdhfebfcbpa...' , it will still be hard for the people to remember them, not unless if they will simply bookmark it.
I think as of now, it was only the school and academic websites are the ones that use an appropriate domain extension which was dot edu (.edu). Even though this domain extension is already common for them but I heard that it was still expensive. Ah well, I guess it is because buyers of it has enough budget and also most are backed by the government.