visa have similar restrictions… the issue is that payment processor, merchant, card network, issuing bank, and acquiring bank (and a few more) are all different layers to facilitating a transaction… there are very few card networks in the world, and essentially people pay online using mostly either just visa or mastercard

technically you could probably run a site that only accepts amex, diners club, jcb, discover, etc but it really wouldn’t go well: people want to whip out their card (often their only method of online payment) and just pay and go

you’d also need to find a payment processor that would be comfortable taking on the risk of facilitating transactions around mastercard and visa… they might just cut them off from access to their network. sounds like extortion? yeah welcome to the world of banking, where in any other industry it’d be called a cartel

making things even more difficult is that by law in the US payment processors must offer at least 3 (i think? it might be 2?) “routes” (these things aren’t always straight forward - sometimes there are multiple card networks involved), so you probably have to have a pretty mature payment processor because they’d have to accept several of the alternative card networks

oh and you have to push various stats to the federal reserve which is a massive back and forth bureaucratic challenge even above the technical challenge of interfacing with some pretty awful systems. it often takes over a year to get things sorted unless you’re a “too big to fail” type customer

there are a buuuuunch of other spanners that get thrown into the works, but i’m not sure where the NDA line hits so i’m gonna just leave it there and say trust me, payment processing (distinct from payment gateway) is an absolute nightmare mess