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.
Once the Bitcoin transaction fee goes up, deposits in altcoin will become more and more attractive for the gamblers. But then, the gambling sites need to tweak their websites and create all the hot-wallets, which is going to take a lot of time and effort, on top of additional expenses. The casino has to decide whether this additional effort is worth it or not.