Though I feel the other big difference is that the Wii U Gamepad was highly limited in its capabilities. You could play games on it while other people watched TV or used that TV for something else, but you couldn't go beyond that. You still had to be close to the TV for it to function, and that usually meant being in the same room.

The Switch being a hybrid console meant you could take the whole thing outside of the room if others needed the space, or bring it with you to events or places outside of the home. So, it solved all the same problems the Wii U did (playing while others used the TV), but also a ton of more pressing ones (taking the game with you while traveling/leaving the house, playing in other rooms/areas, etc).