I think for the most part, at least in the US, it's the acquirers and processors that have set it up that way, sort of a like a "bundled package" (similar to how cable company offer cable TV + internet for $X per month) deal for small to medium sized retailers and businesses.

i.e.: They offer a "VISA and MC" package, while AMEX and Discover are extra features.

Larger retailers like Sam's Club or Costco can leverage their own exclusive deals directly with MC or AMEX.

I wonder if there's ever going to be a US bank that issues UnionPay debit cards. There's not really a need for it though due to Discover working on UP's network and vice versa, but it's interesting to consider.

UnionPay is trying to break into the US market but hasn't managed much yet

OTOH, there is JCB operating in the US as a credit card, is able to run on the Discover network and vice versa and has been here in the US for several years, although cardholder applicants are limited to certain states.

JCB in the US doesn't necessarily partner with a bank though. It operates more like the old AMEX that it acts as its own issuer and network with co-branded partnerships like Mitsuwa and Marukai supermarkets.

They could go that route. I'm sure Union Pay can go a similar route like JCB where they become their own issuer and network in the US, in a limited area and partner with 99 Ranch Market or so.