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.
Casinos would do anything for the best of their clients and that includes spending their time and effort to work on accepting altcoin deposits. Bitcoin fees are a problem for both customers and casino owners and it's in advantage of both o them if a casino has altcoin deposit methods available because transactions can be processed much faster with a lower fee.