Because it sucked...

Bad marketing, long droughts, no consistency, no 3rd party, bad messaging from wii to wii U, lack of ambitious games (color Splash, 2D.5 Mario, sega partnership, etc.

Oh and that fisher price controller. 

Switch is eveything the Wii U should have been and so far... amazing start. This is my favorite NIntendo consoles since the GC. So much potential.

Get rid of that Wii U.

So the WiiU should have had a standard controller with sticks and buttons too small for the average western adult?

Mainly: Software! It flopped because it had no truly epic games. If the Wii U had Zelda BOTW on launch, it would have been much more successful. The 3D Mario was also not that exciting at least not in the first trailer which was shown at E3. The only game that generated some buzz at least was Mario Kart 8 and that came too late, likewise for Splatoon. There were too many generic 2D platformers. I know that they were all great but that's not the type of games that gonna sell your hardware to the mass.
Second reason: Marketing. They didn't nail the point that set the Wii U from the Wii apart. They should market in a away that people know that with the Wii U its a HD console and foremost that you can play all the great Nintendo games on it and this time also all the great third party software like AC 3, Batman and so on. Granted, that statement would have only been true for a short time but people would might have know by then that it is the only console were you can play the best of both world (Nintendo + 3rd Party)

I blame Wii Fit!!!

Bad decisions. They had the best controls in the universe for pointing and have not used them... I've never understood why.

No games.
Poor marketing.
Expensive dev kits.
Underpowered.
Expensive.
Poor online services. No invite feature, no communication
32GB storage for most expensive model which is suboptimal for going digital
Poor services in general. No refunds, no purchases tied to your account, rather on console

Vague name. 

Stigmatized branding. The Wii is frowned upon now. It was a fad that catered to kids, casuals and old people predominantly. Nobody cared anymore as they moved on to mobile (app store/playstore released in 2008), PC or another platform for gaming. 

Well, a lot of this apply to Switch also, but this time Switch has the pros to overshadow it. Handheld form, handheld games, smart gimmick etc.

Wii U is interesting in that it's the main Nintendo system that pretty much would have failed no matter what. Yes, I think it would have failed even if the controller was just the Wiimote.

Wii went from 20.5 million sold to 15.08 to 9.84 million (FY March 2012) even before the Wii U launched .... it was a trend that was clearly losing steam.

GameCube and N64 are systems that I think could have been salvaged to much better sales with a few tweaks. If GameCube didn't look like a damn's child's toy, played DVDs, had Twilight Princess instead of Wind Waker first, and launched earlier, IMO it would've sold pretty decent. If N64 had CD-ROM (could still used cartridges too, Saturn did) it would've won that generation outright.

Continuing the Wii brand was going to lead Nintendo to ruin no matter what IMO.