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.
Spotify is too, just not as drastically. I opened an account in Prague and found when I got to the US that it wouldn't play anything. Curious how they knew I wasn't in CZ anymore, yet instead of just letting me play the US version, it wouldn't let me play anything.
I guess Spotify is still the only game in town :(