N64 and Gamecube says hi! BC made sense with Wii because it was just kind of just an overclocked gamecube. It was also a risk so made sense they played it safe when it came to catring to fans. Made sense with Wii U as they wanted to carry over Wii's 100m audience...
The NX may not feature a disc drive, in which case BC probably won't be built into the hardware and instead will come in the form of digital software emulation down the line.
Wii U doesn't have many games so instead of incurring the cost of emulation, why not make money from the 3-4 games that people will actually want to play and release them as remasters.
Wii U games won't sell NX hardware, so why not repackage of them as NX titles to help push the system and its presence in stores etc.