Casinos needs the wager in USD and not in cryptos, and that's because the wager affects the VIP level, so, if we wager 10 bitcoins while the btc price is in $20k that would mean $200k wager, but if the price goes up to $100k/btc then the wager for those 20 btc would be $2M.
So, if the casinos save the wager in crypto-amounts and not in USD the users would automatically level up their VIP without wagering, they only need to wait while the coin price goes up to the moon.
And that's why the balance in USD or Local Coins makes sense.
Casinos are businesses and need stability, so converting a crypto number into USD will give them that extra bit of control. You're unlikely to see USD swing more than 5% within a day, however that has been shown repeatedly with bitcoin and other crypto. The bonuses and reward levels are not going to be affected by the balance fluctuations because that's not how they work - they only start to accumulate towards levelling up after funds are wagered. Besides that, wagering in USD give users a more stable anchor and reference point, which might be unconsciously and subtly influencing gambler's to instill more faith in their betting.