The availability of music on Spotify is up to the artists and their music label.
If the music is unavailable, the artist or their music label has likely decided to not make it available in 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.
Contacting the artist is a good way to let them know you miss their music in Spotify.
More info here. :)