There were a lot of major problems with the Wii U both conceptually and design wise. If I had to pick the biggest problems it'd be...
* The Name and Marketing - Whoever came up with the name "Wii U" I hope has been fired at some-point, because its one of the most lazy, confusing, and vague names for a console ever. It tells you nothing about what the system is, and makes it seem like yet another add-on for the Wii. Not to mention that the Wii had already lost a lot of momentum by the time of the Wii U's announcement, so it just feels like Nintendo banked too much on brand recognition that wasn't there.
* Bad Hardware Design - The biggest reason the Wii U suffered from a lack of third party support wasn't due to it being under-powered. The Nintendo Switch, all of Nintendo's other handhelds, even the Wii to a lesser extent, prove that even under-powered consoles can still get decent third party support. No, the main culprit was all due to the choice of architecture Nintendo chose. PowerPC and a GPGPU in an era where developers started moving away from custom SoCs and an outdated architecture was completely tone deaf, causing the WIi U to lack all the tools and engines that the other consoles adopted this gen, leading to it not getting even games that could technically run on it. Wii U was designed with an outdated 7th gen mentality that everyone had abandoned by that point and the console suffered because of it. It's no surprise Nintendo went with an off-the-shelf PC-based Tablet processor for its current system vs whatever mess they went with on Wii U.