Don't really know but I can guess. Firstly, everyone thinks the price of gas is too high. You drive to work every day, and gas is basically the only product who's price is advertised from the street! From that perspective. So mentally, I argue, we overvalue an extra 1 percent discount on gas. It's only worth maybe 60 cents a month to me, but worth a lot of other interchance fees for the credit card company.
Secondly, gas stations are a prime robbery target. Credit cards mean less cash in the till. And less chance for employees to steal from the till, and less chance of counterfit money.
Finally, it's a competitive market. If stations don't accept a card, they'll lose business to elsewhere. There's a gas station on either side of an intersection, and you can always tell which station is a few cents cheaper because it's the one with customers fueling up while the other one is a ghost town. They feel they have to compete on convenience or go under, and the credit card companies recruit you into the game with higher cash back rewards.