I don't know what their reason is but what I know is almost all of the gambling site that I played only have bitcoin and altcoin to be used for gamble. Maybe they will use USDT or USD or another stable coin someday, but for now, we don't have to feel sad if we don't see a gambling site don't have USDT or another stable coin at their site.
I tried to look at some casinos and found that USDT actually accepted on bitcasino.io, bitcoincasino.io, cloudbet.com, etc., I'm not vouching for these sites to be legit though, just perhaps these sites are not as popular as the one that only accepts crypto (BTC, ETH, BCH).
I'd rather play with TRX or XRP anyway since it is cheaper and faster.