One theory I think the reason why cryptocurrencies don't allow purchasing thru credit card is because credit company didn't allow them. Purchase  using credit card can be subject for abuse from the owner of the card. For example, the scenario is the credit card holder is compromise or not worthy he/she might purschase a lot of crypts that eventually convert it to cash. The owner of the card didn't not have any plan to pay the transaction made thru credit card. A lot of risk will be burden to credit card company if they allow the purchase of crypts.