Idk why Japan won't buy Zelda more. I mean Nintendo even changed the structure of the game and it doesn't appeal to them compared to the rest of the world.
Perhaps it is useful for me to point out that there are
a lot of used copies of Zelda out there now. The game has been out for a while and many people sold it after they beat it (in Japan you can still get a good payout for your old games, it's not like Gamestop).
So I bet the actual number of Japanese Switch owners that have played Zelda is higher than the sales indicate.
For example. A friend of mine was lucky enough to get a Splatoon 2 bundle. He of course bought the system bundle new but he bought Zelda used (which in Japan means mint condition used). He figured "why not save 1000 yen?" The switch games have no manuals, codes, or discs to scratch they don't even have batteries with old save games to clear. Only thing missing is the shrink wrap. Oh and I guess the assurance that no one might have licked the game card.
Zelda is not a game like Splatoon 2 or Mario Kart, where you want to keep it around for online play or when friends visit. I know there is DLC stuff but a lot of people are just going to play the main game and trade it in for the next big thing while it still has value.
This is of course not true for just Zelda. Pretty much any single player focused game on any system will have this.
I'm sure the number of people who played FFXV is a lot higher as well. Used copies of that game are dirt cheap. Many customers have little reason to buy the game new at this point.
I have no actual numbers to show as no one really tracks used sales in a meaningful way. But just something I thought I would bring up.
EDIT: There are indeed numbers as Chris1964 has pointed out. The most recent can be found here:
http://www.neogaf.com/forum/showthread.php?t=1078514