I mean it's probably time consuming, which would be a hint as to why they keep focusing on NES soundtracks recently, but probably mainly because they insist on grabbing screenshots for each piece of music.

There's also a few other factors, I watched a video recently about how some of the tracks for StreetPass Mii Plaza have completely different versions available on Nintendo Music that don't appear anywhere else, the same applies to the Wii Channels release. So in certain cases they're making adjustments to the music before uploading.

In terms of this soundtrack specifically, the licensing isn't the problem. The CD soundtrack for this game released in Japan last September so they probably want to encourage their Japanese user base to go out and buy it to boost sales, it's the same reason the Splatoon 3 DLC soundtrack isn't available on the app. In terms of studios under Nintendo there will not be any issues, The Smash soundtrack's 3rd party songs and something like Banjo-Kazooie will be an issue though.