Besides, having a wide range of payment methods makes the service look more professional and care about their customers and their confort.
The same apply to crypto casinos. Many of them support too many coins even if some of them will rarely/never be used.
For crypto, usually the casino prefers those coins who have faster transaction speed coz that's what gamblers also wants.
Most gambler, or let's say most people in general wants faster transactions (deposit and withdrawal) so it really make sense for any online casinos to accept multiple coins to attract more players. Though not every coin that has faster transaction speed cost cheap like XRP, but different people do have different preferences.