Network effects. Visa and Mastercard work nearly everywhere (approximately 100% of merchants who take cards in many, but not all, countries). Your big bank can*

Network effects. Visa and Mastercard work nearly everywhere (approximately 100% of merchants who take cards in many, but not all, countries). Your big bank can* issue new cards that aren't on either network, but they need to somehow get them hooked into all the acquirers out there (or at least their payment gateways), otherwise they just don't work. So this is indeed a middleman business, but they have a big moat. * of course, banks don't actually print the cards themselves, there are a smallish number of companies that do that, who may not be set up for your new scheme either, so that's also a thing.