The reason is that the artist or their music label has likely decided to disable playback for it on Spotify.
The availability of music on Spotify is up to the artist and their music label.
Spotify wants all the world's music, and they're working hard on making this happen for you. However sometimes agreements can't be reached with the artist or label or a change may happen in music ownership. 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.
Spotify adds thousands of albums and artists are added every day. If you can't find an artist you're looking for, they may appear (or reappear if removed) soon. In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify.