Compare it to anything else in life. Imagine if you are the owner of a shop and you only accept card payments. Why would you turn down those willing to come to your business with the intention of paying with cash? It doesn't make sense from a business perspective.
Since it's a physical shop where you already fulfill many requirements and follow the regulations, there's no reason to not add more payments.
But, cash and Bitcoin is different, any government will not question anything when you accept cash, but if you accept Bitcoin, you might need to follow certain specific rules. If the rules are harder to met/much effort than the reward, many people will refuse to add the payment.
Do casino hold tokens of the currency they are supporting for their bankroll? I think this is the only reason why some casino don’t want to add bunch of currency.
Yeah, either they or other company that custody their coins.