It's not that. The problem is a lot of people who are racking up credit to borrow for crypto-currencies, then not paying the money back.

Imagine you have a $500 limit. So you borrow $500 and put it down on cryptos.
So then the credit card company asks for $30 to pay for the interest.
Why pay that $30 if you believe that putting that $30 into crypto would make you even more money? Even with all the costs that would rack up through not paying the bill, you would make more money by not paying the credit bill.
The credit card company has to wait a long time before they get the money back and they lose a customer in the "bargain".

Of course not everyone behaves that way, because the answer is that it's not very honourable to deliberately not pay the credit card bill....however, too many people will kid themselves by saying "Yeah but I'll pay it one day....they can wait.."