Good question. Going to take a couple guesses: Probably lock Soundcloud tracks into the Spotify platform? Or have a larger pool for advertising clout?
Isn't some of the draw of SC that the artists know they'll reach an audience? I am not a Spotify subscriber, but I wouldn't think to go to them or Pandora for any unsigned artist - I immediately go to SC.
They might simply be afraid. SoundCloud has all the small artists and all the remixes and DJ sets that are missing in spotify and are already on good terms with labels and artists. SC are planning their own streaming service and once they additionally have the same label based library spotify has they would be a direct competitor.
I just want Spotify to sort out the removal of lyrics