I'm guessing there's a decent chance this is related to the ongoing fallout from the regulations a few years back capping interchange fees for debit cards.
Bank networks can't charge retailers the same high rates, so there's less cream to pass on to the bank customers.
All in all, if that's the case, I think it's a good thing -- not sure why cardless grocery customers should be subsidizing rewards. (It still happens on credit cards, but at least the new-ish debit rules partially have removed some distortion, if even just a little bit.)