As for the "why", the immediate reason is because there have been some legal changes vis a vis how much they can charge to the merchant for the benefit of running this transaction.
More generally, because they want to incentivize people to use their credit cards instead of debit cards. And for those who can't or don't want to get credit cards -- to incentivize them to take their business elsewhere, because the bank really doesn't make any money off people who just have checking accounts. Checking accounts are merely a gateway drug to more expensive banking products.
Doesn't bother me, I get 1% cash back on my credit card and am not dumb enough to spend more than I earn on it, so I already have no incentive to use my debit card for purchases.