it's because with credit cards they can check the credit limit, then be sure that the card can pay the insurance deductible in case of crash
instead with debit i can rent a car, close or deativate the card, crash/total the rental car and then avoid paying any extra fee
most rentals don't rent with debit cards because they want to be sure, and who accepts debit:
- they preauthorize thousands of dollars instead of hundreds
- they only rent the lowest end of the available cars