Things that killed the Wii U:
-Silly name that not only sounded like the anti-thesis of cool, but was misleading to consumers (they though it was a peripheral for the Wii).
-Silly marketing style from the Wii era was kept for almost every major release for TV commericals and the like (unrealistically happy suburban family play games in the living room, ultimately not telling you much about the actual game itself). Sure, you had Nintendo Directs and online trailers that told you a bit more, but the majority of consumers don't see those. Marketing overall just wasn't as aggressive as the competition.
-GamePad. I personally like it, but other than Off-TV Play, it wasn't utilised much. Which is even more jarring when you realise that it was probably pushing the price of the system up by at least $100. It wasn't THAT much cheaper than the competition, meaning it didn't have the "weaker but cheaper system for the kids and casuals" edge that the Wii had.
-Underpowered and unorthodox, making it hard to develop for, resulting in a lack of third party support.
-Lack of a launch line-up
-Even when the Wii U was going stronger than usual, a lot of the games it had were missing core features that previous installments had, making you think twice about shelling out $60 for a new installment in a franchise when you might already have a better alternative for cheaper, or they just lacked industry standard features. Mario Kart 8 lacked a proper Battle Mode and had a less compelling character roster (not counting the DLC because that's MORE money), Smash Wii U lacked compelling single-player content (even the 3DS version does much better at this, with Smash Run and a MUCH better Classic Mode - and it's cheaper and portable to boot) and has a bad online mode (better than Brawl, but still lacking compared to other systems and their games), Mario Tennis: Ultra Smash needs no explanation...
...And hell, even the games that are perfectly fine on the Wii U, generally lacked a "wow" factor. Like yeah, I love 3D World, it's a really solid and fun game. But it has no real spectacle at all to it. You're going from a killer app like Galaxy that made you say "I ****ING NEED TO BUY A WII NOW", to "oh hey it's a fun little Mario game I can play with friends sometimes, cool I might get that sometime". Very, very few Wii U games really had a wow factor to them, especially in Nintendo's established franchises. The only ones I can think of are Smash (but purely on the basis of the roster, once you get past that, it's actually quite a lacking installment - plus the 3DS version has the exact same roster anyway), and Hyrule Warriors. Maybe Splatoon as well. Everything else is just there.
