As others have said, launching without a killer app and then immediately falling into like a 9-month long game drought effectively murdered the system's momentum and got it bogged down in a rut from which is never escaped.

It would have done significantly better if say it had launched with Mario Kart 8 and then had stuff like Super Mario Maker, BOTW, and Splatoon in its first year.

By the time the games finally arrived it was too late and the system had gained the reputation as a console with "no games".

The Gamepad wasn’t the sole reason it failed, but the Gamepad clearly wasn’t a feature that people cared about.

DS (touch) Wii (motion) & Switch (hybrid) were all able to find such massive success because their unique features made gaming more accessible for people. Nintendo lost sight of that during the 3DS/Wii U era and went with “new gimmicky feature for the sake of new gimmicky feature.”

Essentially Wii U was a system that tried to retain the “casual” Wii audience while also attracting the “hardcore” PS/XB audience but instead wasn’t appealing to either crowd.

Like mentioned above, Wii and its motion controls made gaming more accessible to demographics that weren’t being served by PS/XB. Wii U got rid of the Wii Remote as the primary controller and replaced it with a traditional dual stick controller with a screen on it, pretty much the antithesis of the Wii Remote and thus did not entice that audience to migrate.

As for trying to attract the PS/XB audience, PS3/360 were 6-7 years old when Wii U launched so releasing a more expensive console that is in the same ballpark in terms of power with a handful of AAA 3rd party ports was never going to make those people switch over, especially when the significantly more powerful PS4/XBO were just around the corner.

Another major aspect was 1st party software. Wii U launched with NSMBU & Nintendo Land, their thought was probably NSMB sold over 30 million on DS & Wii each so launching with a new one is a no brainer and Nintendo Land shows off the Gamepad features so it will be the new Wii Sports.

On paper, that sounds good but as I noted, people did not care about the Gamepad like they cared about motion controls so Nintendo Land had no chance of being the new Wii Sports. As for NSMBU, it’s a great game but it was the 4th “New” title in a 6 year span (with 3DS getting a New title just 3 months prior) so it was no longer “New” and couldn’t carry the system on its own.

Then we have to look at post-launch. The poor sales quickly led to 3rd parties dropping support and the transition to HD was hard on Nintendo, the combination of these meant Wii U would often times go months between major releases.

To summarize, Wii U was a system that tried to be a middle ground of Wii & PS3/360 but failed at appealing to either of those demographics and instead just became a system to play a handful of major Nintendo IP (which for many people 3DS was good enough for that).

let's be real, it was absolutely the game pad.
No reason to retcon the obvious truth.

The game pad was a pure gimmick, a gamble that the big thing that gamers wanted was a way to keep playing when someone else in the family wanted to use the TV. While I'm sure that's an issue for some people, it's not an issue for many people, and that seemed to be the whole reason behind the entire design of the system lol. It was all just extremely poorly thought out.

And yes it made it so you have this awful awkward asynchronous gameplay where one person has the game pad and everyone else uses last gen controllers. I remember when hearing about the system before it came out and finding out it's just a single giant controller and all I could think was that's a bad ridiculous idea lol. While N64 was THE multiplayer system of its day, and GC was a good multiplayer system, and Wii had fantastic casual multiplayer games to bring everyone together for a night of gaming, the whole design of the Wii U screamed "this is a single player console only and it's gonna be awkward weird different controllers asynchronous gameplay if you want to do multiplayer".

And then of course it jacked up the price of the system and hurt its specs all for the sake of this gimmick nobody wanted. Whereas the Wii saw Nintendo stop chasing power to do a novel fun new way to play games that was also affordable, WiiU saw Nintendo do low power again but just to make a far less affordable system with a bad gimmick that did nothing to create novel fun new experiences. The whole async multiplayer is something that maybe a handful of games could work around, it's not what you base a mainstream system design around.

So the game pad made the whole concept of the system awful, jacked up the cost, kept the performance low, made multiplayer weird, and introduced nothing fun and exciting, all for a gimmick few people wanted or found useful. Add to that the fact that nobody was making games for the thing - probably because consumers and companies alike were like wtf when they saw the system so everyone knew there wasn't gonna be much market for it's games. And the cherry on top being that it was a successor to the Wii but years after the whole crazy Wii fad had died out, so the one thing it tried to have going for it was already over several years hence by that time.

So yeah, it was absolutely primarily the game pad.

I agree with you, mostly. The lack of a killer app was a major reason. A good killer app, like Super Mario Bros, Wii Sports, or Breath of the Wild, can drive massive sales. But, I don't think these games translate 1:1 in success over to the Wii U platform. The Switch had some big advantages over the Wii U in terms of accessibility and ubiquity to playing multiplayer games (basically everything the Wii and 3DS had in one, with with more advanced engineering than both).

So, I agree killer apps are necessary, but IMO a killer app is hardware context dependent. A good example of a system seller on one platform, but not another, would be the Tiger Woods on Wii versus other platforms; I doubt anyone was buying consoles for Tiger Woods games; but people did buy the Wii for it. With Mario Kart 8, I think the handheld, being able to congregate and play outside the home (office or school space), and having 8 or 12 player multiplayer (12 with a WiFi network) makes the game much more desirable than what the Wii U offered with the split screen.

I guess what I'm saying (using the Mario Kart example): Mario Kart 8 as software isn't a killer app on its own, it depends on the context of the console it's on. if Mario Kart with the Switch features is something players really want to play, but Mario Kart with the Wii U features doesn't appeal to a sufficient number; then, Mario Kart Switch is a killer app, but Mario Kart Wii U isn't.

In short: Killer apps are very useful for driving a lot of hardware quickly. But killer apps aren't just the software, but their ability to make a player want to play that hardware.