My point (theory) is that Mastercard isn't offering them a big enough volume discount. If they could pay 0.1% I doubt they would exclude MC even if it meant losing out on some Costco credit card accounts.

I'm saying excluding other card brands gets them a bigger discount from their exclusive card network than volume alone would, not that it pushes people to the Costco-branded card from that network (though it probably does that, too.)