The Wii U was an amazing platform specifically because of the second screen. It lended itself nicely for asymmetric multiplayer (Rayman Legends is so good on the Wii U) and the ability to play without using the TV is also pretty nice.
Yeah marketing was certainly an issue, but the other issue is that the Wii was mostly owned by casuals and casuals don't need to upgrade ever.
Hence why the Wii received it's last game (Just Dance 2020) a whole year later than the Wii U received it's last game (Just Dance 2019).