There is another group that cannot use credit cards:  People living outside their own countries.  Credit doesn't follow you outside of your home country.  If you are visiting on a temporary basis you may pay it when you get back or over the Internet but in the long term credit cards are not convenient for say an Indian Exchange Student studying in Australia.  How does he pay his credit card?  He must leave a large amount in India that he might need in Australia or pay large fees to transfer if he needs to make a purchase over the Internet.

Now there's Bitcoin.