While gift cards are cool, the actual credit cards have gotten a lot of restrictions over the years. At least in terms of online casinos.
In terms of online gambling, regular credit cards are riddled with multiple chargebacks and fraud. People make a deposit with a credit card, gamble way too much, lose and then spend more trying to get it back.
To avoid those sorts of scenarios, casinos are better off denying actual credit card users to deposit funds and look for other sources. Specifically, those that have a fixed amount like gift cards or other accounts that are hooked to a bank account.