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).
When the herd loses its way, the shepard must kill the bull that leads them astray.
