The exclusives weren't that great.
Super Mario 3D World was one of the worst 3D Marios they could have put out. We went from Galaxy to that and it hurt. There's no mainline Zelda game (they'd delay BOTW to the point where it's a Switch game). Retro was forced to make a boring DKCR sequel instead of an awesome first person shooter or WRPG. No Metroid. No third party support (even the GameCube had some decent third party games). Smash Bros. 4 seemed like such a weak attempt that the name was literally "for Wii U" as if it was a commodity.
The tablet controller appealed to no one. I predicted the Wii U would fail immediately because the tablet controller made no sense. The Wiimote was obviously a hit, but Apple was trying to hop on the tablet bandwagon at the time but it literally added nothing to the system. I mean, it wasn't even like a console-DS. Nintendo would go with the "last gen graphics" but charge a full gen price with NO benefits. The Wii could have weak graphics because of its mass market appeal with motion control. Same with the Switch which has the portable gimmick. The Wii U had NO reason to be so weak. It didn't need to be PS4/One strong, but it should have been like PS3.5 or Xbox 540 strong at least!
Lack of online features in 2012.... in 2002 it was acceptable, but not in 2007 much less 2012! The price was way too much (as mentioned above). No hard drive. I could go on and on and on and on and on.
The Switch thankfully rectified (or is rectifying) a lot of these issues and is an amazing console . The Wii U was just a bad dream.