I suspect it's because even if it's what customers want (which, again, is not a binary thing), it ultimately doesn't make a big impact on revenue either way (arguably there is a long-term second order effect on revenue in terms of the health and longevity of their employees, health insurance premiums and payouts, etc.). And casinos seem to really like inertia and doing things the way they've always done them (though some of that I'm sure is for regulatory reasons).

So whether a casino is smoking, or non-smoking, it has little short-term financial incentive to switch over to the other side. Not because customers want it one way or the other necessarily, but because it's not a make-or-break criteria by which most customers decide whether or not they want to go to the casino.