The thing is, at first, it wouldn't have made sense to use debit cards. Banks were small, and rather fragmented. If you wanted a form of payment that would work everywhere, you needed to go with one of the credit card companies.
It's hardly the case now, though. There are already debit cards which work nationwide in most developed countries, and international ones are starting to turn up. Once these are readily available and in use, is there any reason that credit cards should remain in use?
Well, unfortunately, probably yes. People seem to have gotten to like running up vast debts on them. That they are about the least sensible legal way of borrowing money doesn't seem to cross the mind of people whose credit card debt exceeds their annual income.
For that matter, why are they even legal? I seriously doubt that it'd be legal for a bank to lend someone 20,000 euro at 15% interest under normal circumstances, but it can do it quite happily on a credit card. Odd.