Network effect / moat / unique offering?, page-14

The payment is the facilitator. Why do stores accept Amex and Diners when visa MC are cheaper?

It's because of the customer. They don't want customers to walk away for an extre 0.5% fee.
APT is not expensive at 4-6%

Why sign up for a new account when you already have an APT account? I'm not going to get myself diners card if a site only accepts diners. I will buy from someone else. Same for venues which only accept cash. I will go elsewhere.
I think you are looking at it idealistically rather than through a socialogical lens.
eg: People buy horrible, ugly, expensive cars. Because they want to. Not because it is the best option.