I don't like using the Gamepad. I hate how slow the UI was. No quick resume or standby like other modern platforms. It just felt slow and clunky and unwieldy.
As for developers. Wii U used a modified CPU from 2000. A 12 year old CPU worse than the one found in a 2005 Xbox 360 in the year 2012. That's a problem. Wii U only had 1GB of ram for games in 2012. That's a problem. Wii U used Nintendo's API instead of something more standard. That's a problem. UI was slow and could not multitask. Wii U used a GPU from 2008 in 2012.
Switch uses a much modern modern hardware. One that devs are familiar with. 4 times the ram and I think faster than Wii U's. (not 100% sure on that) More powerful than Wii U. Used Nvidias API in NVN which supports OpenGL and Vulkan. No question Wii U had shit marketing and name where Switch was the opposite but hardware played a big part as well. Not saying Switch online is the best but still better than Wii Us.