Zelda's never been Nintendo's best-selling franchise, but it's almost certainly its most well-respected, particularly among the "core gamer" audience. In addition to Ocarina of Time still topping "best games of all time" lists twenty years later, even an unloved entry like Skyward Sword still got a 93 on Metacritic and sold 3.5 million units in its first month on a "dead" system.
Even if it doesn't have the mainstream appeal of a Mario or Pokemon game in terms of sheer unit sales, it gets them tremendous word of mouth. The Zelda brand has huge prestige, and Breath of the Wild has done a tremendous job advancing it. If Sun & Moon had been Switch launch titles, sure, they would've sold very well and people would be enjoying them. But they'd just be new, good Pokemon games. Right now the story people outside of gaming are hearing about the Switch is that it's home to one of the greatest and highest-reviewed games ever made.
