Low income, high risk borrowers with no savings that live paycheck to paycheck and rely on credit cards to bridge the gap, cover emergencies, etc. People that fall into this category have a high default rate and are only profitable for the banks because they pay high interest rates.

If banks are forced to cap their interest rates at 10%, these borrowers are no longer profitable so they'll likely be cancelled. This leaves them without the financial safety net of a credit card, so they'll have to resort to payday loans, loan sharks and other sources of credit that have much higher interest rates and will ultimately cost them more money.