Need help? Check out Spotify Answers for solutions to a wide range of topics.
The availability of music on Spotify is up to the artists and their music label, so the reason is that the artist or their music label has likely decided to remove it from Spotify.
Sometimes the music is only available in certain countries, while access is blocked to others.
Sometimes the album or song will be unavailable temporary, and come back again later.
Hopefully it will come back later.