The problem here is interoperability.
Now most merchants have to work with two companies, visa and mastercard. Want to accept russian MIR cards? Well, in some countries you're not allowed to, and in some, you must, since visa and mastercard don't work there. Now if you add a european company to the mix... whill their cards get accepted in south africa? What about in eg turkey? China? Will whatever indian alternative is get accepted in france?
Currently, with a visa and mastercard, except for maybe russia and iran, you're pretty sure it'll get accepted at least somewhere in any urban area you visit, so you won't be hungry and have somewhere to sleep. If my bank replaces my mastercard with the EU alternative, I won't be that confident about that for quite a few years.
On the other hand, cash is still the king of everything everywhere... somehow some politicians are trying to get rid of that for some reason.
Most merchants don't work with Visa and Mastercard, they work with payment processors like Fiserv, or other middle men even further removed from the card networks, that already abstract all the different cards (including existing local debit cards) away into a unified flow.
Sure, there's a gajillion of those processors all over the world, and they somehow all work with mastercard and visa.
Diners club? Well.. "it depends". Many don't work with it.
Indian, russian, chinese, cards? Maybe in india, russia and china, but you shouldn't expect it to work "everywhere" like visa and mastercard. Same will be true for EU cards for quite a few years, especially if the system gets fragmented into many different companies using many different systems, and you'll always wonder if your german card will get accepted in Algeria like your friends' french card is.
> Want to accept russian MIR cards?
Thankfully, this use case has been solved by Russians themselves.