There are new coins coming up created for gambling alone like Eos, but the majority of the gambling sites still prefer to use Bitcoin and other top coins in the market, because of the liquidity factor, but I agree that gambling sites should offer more altcoins to use by their players according to their preferences and conveniences.
Eos has not enough support and userbase behind it in order to become a major player in crypto gambling although it is created exactly to do that.The reason crypto casino stick with bitcoin is because it has the largest userbase and many new people are introduced to it daily.This will not change easily.
Offering more altcoins will for sure enlarge userbase of crypto casino. I agree that bitcoin still dominates in crypto gambling but situation may change in the nearest future.