I feel like you've never actually worked in any form of marketing. People talk with their wallets, if they aren't investing, then a company takes it as lack of interest. If there's no interest from the customer, then there is no interest from the company. They aren't going to invest money into something, if they aren't going to get money out of it. If people wanted Wii U support on the Switch, they should have been buying Wii U games in the first place.
Expect in the case of the Wii and GC, they weren't making a massive investment into backwards compatibility. Adding backwards compatibility wasn't a struggle when the systems are using the same architectures.
The Switch and Wii U are completely different systems with completely different architectures. Setting up Wii U games to work on the Switch would be a costly investment. Something that isn't worth it for Nintendo