Why it isn't uniform is simple: It depends on who does their card processing and what special deals they offer the merchant.

While Visa and MC are almost interchangeable in the USA and Australia, I've encountered quite a number of merchants who accept only one or the other in Asia and in Pacific islands. (The most ludicrous example that I can remember was that the ANZ Bank in Fiji would not accept MasterCard ... not even when issued by the ANZ bank itself!)