Because FIs are in the business of making money. Sure a devout Muslim can get a regular CC and pay it off at the end of the month. But isn’t the use of the credit facility which is funded by interest charges and merchant fees at the root of halal versus haraam credit facilities?
A credit card with an annual fee allows the FI to make money despite the fact that they are not charging interest on a balance. It also goes to the adminsitration costs of offering and maintaining pre-paid credit cards.