People knows that Bitcoin is good for investment and not for gambling that's maybe the reason why people always prefer other altcoins or just used USDT because they know that this is more convenient crypto to use for this matter.
The BTC network now has such a low transaction fee that it would be more profitable to use BTC for deposits or withdrawals on gambling platforms than USDT. I think one of the reasons why gamblers still prefer alts more often is that gambling platforms and exchanges often set unreasonably high fees just for BTC transactions.