Because Banks that issue your credit card has its processing time before it pays, I think 2-3 days is the minimum and most of the exchanges that allows it will confirm it first before they will release your bought coins so it means that the processing itself will take days and those exchanges is making sure that those card is real and will not reverse its payment.

And I think those banks that are issuing credit cards is surely monitoring to where and when will you spend your balance as it is not fully paid, maybe when they know that you will buy some cryptocurrencies they will delay it and will give you more hidden fees. We know that banks really hate cryptocurrencies though.