Using debit cards means that you need to keep a sufficient balance on a zero interest checking account in order to make transactions.
Using credit cards allows you to keep close to a zero checking account balance and manage your own cash flow, since credit card bill dates are deterministic.
Why would anybody want to keep _any_ amount of money in a non-interest bearing checking account right now especially when the risk free rate of interest (US treasury bills / equivalent money market funds invested in US treasuries) yields 4.00%+ APY now?