That ship sailed when they elected to not include upscaling in the Wii U's Wii BC.
This is just piggybacking off the hardware that's already there.
There is upscaling on Wii titles, which is the only reason it can output in 720p or 1080p. Just not upresing, which would require additional testing on a game-by-game basis in order to make sure everything performs well at the higher rendering resolution. And many titles also probably used low-res textures, so they would look out of place with higher resolution geometry. Upresed backwards compatibility just isn't feasible.