It was not only the gamepad, that's for sure, but the thing drived the cost of the system up so much that it became a problem, and it was rarely used very well.
Other problems were the hardware itself, the PowerPC architecture was just too outdated and weak, even if the system otherwise was capable enough. The name also sucked, which was an issue. Most importantly though, the marketing was a mess!
It's a bit of a paradox really, Nintendo couldn't sell the gamepad idea, but if you don't have the gamepad you are essentially just selling a PS3 in 2012, but without the multimedia functionality. That'll never work.
The gamepad caused the Wii U to fail, but it was also the key for potential success, had they sold the idea well. They needed more games that showed what the idea could be used for. Nintendo Land and Mario Maker were great, but there were just not enough big titles like that. The biggest missed opportunity was exploring the non-gaming features - having Spotify, Netflix and all the other popular streaming apps that were booming at the time could be a big win for a system that can so conveniently be navigated with a tablet interface while natively connect to a big TV and a sound system. I know Netflix was there, I really enjoyed using it that way, but I saw very little marketing of that feature, and most other big apps were missing.







