The difference is that since cryptocurrencies' security is based on PoW, the more cryptocurrencies that are accepted by large numbers of merchants, the less secure each one is against attack.

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.

There may be alternatives but just one is definitely preferable.