Firstly, In the early days, casinos paid their players as credit card refunds. The fees for the credit card transaction were refunded too. (Credit cards were never set up for merchant to pay a customer.) So, because casinos were abusing the system, they were having the credit card company do twice the transactions (deposit and withdrawal) for no fees.
This did not endear them to the credit card companies.
Secondly, gambling transactions have a high rate of both fraud (actual stolen card use) and buyer's remorse (chargebacks). Those eat up security resources at the credit card processor's end. We all know how carefully they screen deposits (as opposed to withdrawals.)
This did not endear them to credit card companies.
Thirdly, as US credit card companies decided to decline processing transactions for gambling at all, third party processors popped up who would lie about the nature of the transaction in order to push processing through.
This did not endear them to credit card companies.
Fourthly, the Patriot Act and other anti-money laundering bills and regulations were passed that add to the security recordkeeping, and the DoJ started putting pressure on advertisers etc.
So the credit card/ debit card companies in the US are out of the gambling business. It isn't all their fault and it isn't all the D0J's fault; part of it lies on the online gambling industry itself in defrauding them in the early days and in doing bad transaction quality screening.