I copied this from the music faqs:
The music / artist I'm looking for isn't on Spotify. Now what?!
We want all the world’s music on Spotify. However, some artists and tracks are not currently available. Sometimes agreements can’t be reached with the artist or label or a change may happen in music ownership.
Thousands of albums and artists are being 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.
You can also open this form and provide us with feedback.
Hope it helps you. :)