Because it's garbage. Shitty controls and revisiting that one dungeon over and over sucked balls. Nintendo was masturbating themselves with the stupid touch screen controls ruining so many games. Prime Hunters,Zelda,Mario 64 DS all suck for the horrible awkward controls. Hourglas more so for just repeating the worst Zelda dungeon ever so many times. Gave up on that game,sold it never looked back. Nintendo is capable of making Bad Zelda games and that is their worst. Triforce and such are spinoffs. No one looked at those and said ah yes the next mainline Zelda is coming. It's a sub series within Zelda.
There are 19 mainline Zelda games and all games that start with "The Legend of Zelda" are mainline and part of the timeline. Zelda II is the only exception. The only games that are spinoffs are the Hyrule Warriors games, those weird Tingle games, Link's Crossbow Training and Cadence of Hyrule. All of the multiplayer Zelda games still have the same basic gameplay like all of the other mainline Zelda games.
And I heavily disagree about touchscreen controls being terrible.