The artist or the label the artist's content is on needs to add it to the Spotify service. All content on the Spotify service is up to the artists and labels if the content will be available for streaming.
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.
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) soon. In the meantime, feel free to contact the artist or their label to ask when they plan to be on Spotify.