There are a whole class of people who can't get credit cards, for example young people, students (in Europe) who have no regular income. Poor people who are too poor to get even a bank account, in developing countries. Illegal immigrants who can't go to a bank where they live. People with bad or no credit rating, ...

The list can go on and on.

This is a sizable chunk of the market which is currently not served by regular credit card companies.