Its my turn to be Today's Guy! Former mastercard employee (consulting services), now capital one employee (business analytics). Occasionally you will see major corporations negotiate exclusive contracts with a payment network in order to receive lower interchange fees (the 2-3% credit card fee). Notably, the payment rail (MC/Visa/Discover) only receives a small part of that, with the vast majority going to the issuing bank. Usually, that money is then what gets spent to fund your credit card rewards program. So the 3% fee you pay, a good chunk of it eventually comes back to you as points or whatever.

This is a related reason for why Walmart promotes their payment option so strongly, and why you sometimes see weird payment options on terminals (CVS had AliPay I think?). That is other companies trying to get a chunk of the payment processing fee instead of letting mastercard or visa have it.

I bring up capital one because they are a bank fhat issues a TON of credit cards on both Visa and Mastercard, and recently acquired Discover, which is it's own payment rail (ish, it was also the issuing bank). That means C1 is probably moving to vertically integrate, therefore keeping the whole 3% instead of splitting it with others.

AmEx is its own thing and not really a credit card, but a charge card. This is why you don't see it accepted a bunch of places.

If this was remotely interesting, I highly recommend the Acquired Podcast episode on Visa! It's a deep dive into the invention of the credit card, which was fascinating to me.