Do you know the reason why this is happening ?
I think it's because of the frauds that are taking place. Some people launder money, some people deposit illegal money etc...
May be it was becoming difficult for those payment gateways to distinguish between such activities and so they are considering to restrict it.
I believe the most likely reason is because of regulatory pressure. It does not necessarily mean an unfriendly regulatory outlook but rather a more enforcement of strict policy for the cryptocurrency gambling business.
In regard to fraud taking place, certainly the services already consider such thing as possibility within the day-to-day business activity, hence, they already have an AML policy set in place for their own platforms and services. So this one can be ruled out from possibilities. You can take a look at the related policy on their site:
https://www.moonpay.com/legal/aml_kyc_policyhttps://transak.com/terms-of-servicePerhaps these fiat- crypto payment gateways are ditching casino businesses because it might seem that they have less profit and more regulators bearing down as compared to others who don't focus on casino businesses at all.
With the rising crypto gambling interest both from the players and the casino, I doubt those payment gateway services did not find it lucrative.