Spotify wants all the world’s music on Spotify. However, some artists and tracks aren't currently available (in a certain country). Sometimes agreements can’t be reached with the artist or label or a change may happen in music ownership.
However, please know thousands of albums and artists are added to Spotify 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. Thanks!