Maybe because what it represents stings more than the actual inconvenience.
Nintendo's initial plan was to use Zelda to save the Wii U, and promised Wii U purchasers its exclusivity. When zero games, from Mario Kart to Smash to 3D World, could turn the Wii U around, Nintendo realized it'd be pretty dumb to waste all these assets just so a few people who really, really, thought Nintendo wouldn't let them down wouldn't be let down.
Pretty shitty thing to do, but capitalism is capitalism. Let's see if it works for them (it will).
BoTW delays could also simply be due to the game's size and scope?
Nintendo probably completely underestimated the development time needed for an open world game, and as they were filling in the world I think they started to run wild with possibilities and mechanics.
When Aunoma made the announcement it was going to be delayed from 2015 to 2016, that's probably around the time they knew it was going to come to the Switch (NX) and had extra time to pad out the game. It's going to be a really massive title, and I feel like not launching on Switch is a missed opportunity...
What do you think about the rumor that a full fledged Mario title will be a day 1 Switch game, is there an overlap with the Zelda hardcore fanbase in that this demograpic would have a Switch before Zelda anyway because of Mario's release? That's another key difference, Wii did not launch with a Mario title and neither did Wii U, both came a full year later for those systems.
Also be prepared for the cacophony of Nintendo apologists for making that Wii Sports argument!
A 3D Mario on release day would probably fill the hypothetical void BotW would leave. Even though we've even seen snippets of the 3D Mario, I still feel BotW is a more stunning achievement, and one that really revolutionizes the Zelda franchise and pushed the open world genre forward. 3D Mario won't excite a certain audience missing on Nintendo's platform the same way. I also have lofty expectations for BotW, I think it will push atleast 8 million units.
Switch will likely have a fantastic launch, and Zelda 3 months later in June would give the console some serious momentum.