Also, Mastercard and Visa are both spending the same currency. If you have multiple cryptocurrencies, the exchange rates would fluctuate with each other and it would be very inconvenient for the user.
It's not really that much different than a website that accepts multiple global currencies now...like Dollars vs. Euros vs. Pounds vs. Yen, etc. They could opt to base their sale price on the most stable crypto and make conversions on the fly like exchanges do now when they show the price of one crypto based on the units of another crypto. Most points of sale could be programmed to do this in the future without too much trouble.