Probably license agreements with the labels... but that's pure speculation. Spotify seems to have solved that problem by throwing tons of cash at them.

You can see here[1] that Spotify has made agreements with specific labels for international rights. What a thing... only licensed to stream a song in some select set of countries... absurd in 2020/2021 and being an internet based service - but that's record labels for you.

[1] https://www.theverge.com/2020/4/1/21203324/spotify-warner-ag...