Ok so lets go back to say the NES... you had your hits (and amazing 3rd party games). Same with SNES N64, and started off strong for the GameCube, and some decent games for the Wii and Wii U.
I bought a Gamecube just for Wind Waker, played it and returned or sold the system (can't remember which). Wii and Wii U have about 2 or 3 games I would play on the system and then let it sit and collect dust.
Nintendo consoles (especially the Wii) were great for young kids, elderly and party games. But both my nephews (yeah my awesome demographic) loved the sports games on the Wii (bowling to out of the park baseball), but Daddy has a PS3 360 and a PS4, and now they are totally into the sports/driving games on the big boy consoles because they are more closer to the sports they play (they are 7 and 11 and into soccer, baseball, basketball).
And Nintendo's first party games are generally pretty good..... there isn't enough content for a gamer who grew up with Nintendo to keep going back for 1 or 2 games. And no matter how badly Nintendo is trying to market this to 20-30 somethings on the go (or that horrible e-sports part at the end), adults will stick with a tablet, smart phone or laptop. Why would I want to carry around another device, especially one with detachable parts (and you thought loosing a 3DS stylus was bad)?
They are creating a market that's half-way between mobile 3DS and full blown console like the big boys but not hitting the mark.