I do agree that Ripple is centralized and many people do not like xrp but remember this is a gambling site. If someone has ripple he can directly deposit them in stake.com to gambling, rather than first converting them to bitcoins and then depositing. The more options of deposit the gambling site give, the better and easy it is for the gamblers.
Not only Ripple, but also any altcoins that have cheap transaction fees, and fast transaction speed might be added in gambling sites sooner or later. Because they will help both casinos and gamblers. Additionally, there are other factors such as total daily liquidity, and history of network congestion. Casinos and gamblers need as fast transactions as possible, so it will turn out to be so terrible if network congested.