I suspect this is an issue of "We have x legacy Mastercard customers. It will cost y to add ApplePay to our system. The LTV of X < Y. Hope they don't mind, too much."

For me the big question: when Costco shifts to visa, will they also accept my PP MasterCard? Already it is the only reason I'd ever spend money on my Amex Plat, aside from giveaways like the $200 airline fee and SmallBusiness.