Correction, it works with Apple Music as well[0], and with local music files[1].
As for why, it seems like those were just the only two that bothered to integrate their apps with Playstation. Which kind of makes sense, as Spotify and Apple Music are the two largest streaming services currently (by user numbers[2]). Youtube could arguably claim to be more popular, but their YT music app feels half-baked even on the primary platforms (iOS and Android). And the other ones right behind are Tencent Music and Amazon Music, which are rather unpopular, at least in the US.
Weird framing of those linked numbers: Spotify (30%) is way out in front and Apple (13.7%), Tencent (13.4%), and Amazon (13.3%) all have similar market share.
As for why, it seems like those were just the only two that bothered to integrate their apps with Playstation. Which kind of makes sense, as Spotify and Apple Music are the two largest streaming services currently (by user numbers[2]). Youtube could arguably claim to be more popular, but their YT music app feels half-baked even on the primary platforms (iOS and Android). And the other ones right behind are Tencent Music and Amazon Music, which are rather unpopular, at least in the US.
0. https://www.playstation.com/en-us/support/subscriptions/how-...
1. https://www.playstation.com/en-us/support/hardware/play-vide...
2. https://explodingtopics.com/blog/music-streaming-stats