Casinos want variety for payment system so that their games will be easily patronised and so they add dogecoin because of the fees that is very good for gamblers. Not every gambler have bitcoin because some trade dogecoin and transfer same to gambling account to play. So it is still in use as option for some gamblers.
In fact, the casino makes no difference which payment system to use, the main thing is that it be reliable. In fact, all the risks when using cryptocurrency (in terms of price) lie with the player - if the cryptocurrency depreciates 10 times, then the player will suffer first of all. For a casino, it doesnt matter where they take their margin: from $1,000, from Bitcoin for the same amount, or from Dogecoin for the same amount.