Well, there was talk of Wii U's being recalled in December and being hard to find on store shelves so Nintendo may have been making sure that Wii U's were off the market to push more people to purchase the Switch for Zelda. Of course, used games stores like Gamestop / EBGames offered discounts on Switch consoles if people traded in their old Wii U's so there probably would be some Wii U's available but not at a large quantity. Furthermore, it seems that Nintendo has published far fewer Wii U versions of the game limiting the availability of stock for the Wii U as well. Thus, I think there was some smart planning and some luck involved.
I find the Switch launch interesting compared to the Wii. Both systems had a Zelda title at launch which was also released on the previous console at or close to the same time. Even though Twilight Princess received a lot of praise and was a popular launch title choice for many users, it was Wii Sports that had the buzz and basically sold the Wii console for a year or two even. This time around, Zelda is clearly the killer app and the reason people are purchasing the system. Even if 1-2-Switch was packed in, I doubt people would be buying the Switch for that title. It would still be about Zelda. In that regard, along with other changes, BotW and the Switch launch seems more comparable to the Nintendo 64 launch to me right now than the Wii. Yes, the mobility of the console has gamers interested like motion controls but the coverage and press and Zelda is getting right now seems to be a bigger motivating factor as well as the fact that the retail line-up of games to get released for the system for the first little while is a bit low like the N64's launch but more good stuff will be coming.