Spotify launched on iOS. Apple saw them competing with iTunes and basically stole their idea to compete with them. That, alone, would be okay. But Apple Music is privileged within iOS and Apple's marketing in a way Spotify cannot be.
a) Spotify didn't invent music streaming. There were many services e.g. Pandora that were doing in the years before. It was a pretty obvious idea once devices had faster bandwidth.
b) Apple didn't steal their idea. They acquired Beats who had launched a similar service soon after Spotify.
c) Apple Music isn't privileged. It comes pre-installed but otherwise you can delete the app and use Spotify, Youtube Music etc.
Any functionality that Apple Music allows over Siri that Spotify does not is, at this point, up to Spotify to implement.
Google does the same thing. Unfortunately, with the near zero marginal cost of software, I do not see any way around vertical integration unless the law started arbitrarily segregating businesses.
Also, Apple Music came out 5 years after Spotify, so it had a pretty healthy lead. But regardless, any non Apple vendor competing with Apple's bundled products is going to face an uphill battle.
It predates Dropbox by almost half a decade