How much were you charged?
There is a charge for processing debit cards, although it is nominal in comparison to that charged for credit cards. Most banks will charge a percentage of the transaction fee for credit cards, usually around 2.5%. Where as debit cards are generally a fixed fee which can be anywhere from 10p to 50p.
So with low value transactions, debit cards can actually be more expensive, where as credit cards are far, far higher for anything over say £10. Most companies choose not to pass the cost on to their customers as, like bingofuel says, cheques are far more expensive and customers often don't have cash.