I don't see what you're not understanding. If you live in America, then surely you are aware that Visa + Mastercard are overwhelmingly dominant as payment networks, yes?

Obviously Discover, Amex and Diner's Club exist, but they are barely worth mentioning. Hardly anyone (proportionally) carries those cards, and only a few merchants accept them.

In the United States, for a merchant to refuse Visa would be suicidal. In Canada, it's almost a non-issue because all the debit cards are processed on another, non-Visa network.

What of the above do you disagree with?