Why did the Wii U bomb so hard?

Bad marketing calling it the Wii U, people got confused thinking it's just another Wii or something

and no 3rd party support pretty much

Sad thing is Nintendo and a few companies did release a good set of games on it, it could play all Wii games, and had a decent virtual library

It was a combination of godawful marketing and mistimed release window.

There were actually plenty of good games on it, people just don't know about them.

The fact that it released 'half-generation' was a problem; the gimmick isn't unique enough to take over the casual market the way the Wii did, and the enthusiast market looked at the specs and were unimpressed (I could rant about how they're idiots for caring, but whether or not I think they're idiots, it has a noticeable effect.)

Right out of the gate Nintendo did not know how to market it. Their ads did not make a proper distinction between the Wii and the Wii U and as a result a LOT of people assumed it was some kind of addition to the Wii and not a separate console confusing and turning a lot of people off of it. On top of the poor marketing the Wii U didn't have a starting lineup of good first party games at launch or releasing during the first year. That then contributed to a lack of third party support for the entire lifespan resulting in a failed console. The Wii U did eventually end up with a good number of solid first party games (smash, mario kart 8, splatoon, bayonetta 1 and 2, Mario 3d world and U, Mario maker, xenoblade, Pikmin 3, Windwaker HD, Donkey kong tropical freeze, Captain Toad) but it was already too late.

The Wii U is a great case study on how not to release and support a product while the Switch is a great case study on the opposite. Nintendo made it very clear what the gimmick of the switch was (portability) and built a lot of hype. They had amazing first party support in the first year with the biggest IPs being Botw (for launch) and Mario Odyssey (for Holiday season) further increasing console sales. On top of that we got a plethora of third party support (dark souls is gonna be on a portable Nintendo console) as a result of the first years success.

Mostly game choice and it didn't really have anything on the wii other than smash 4.

Because it's a bad gimmick.

There wasn't enough quality games on it, which kept people from spending money on it, which kept developers from producing quality titles for it. Also, while the WiiU released and was their new flagship console, 3DS got significantly more attention from developers, including the "new 3DS" release.