The relevant FAQ says:
"We'd love to have all the world's music in our catalogue. But for a number of reasons some artists, or a few of their tracks, aren't currently available on Spotify. This might be down to agreements not yet having been reached with the artist or their record label, or due to changes in ownership. We're adding thousands of albums and artists every day. So if an artist is not currently present they may well be appear, or reappear, on Spotify shortly. In the meantime, you could contact the artist, or their label, and ask them if and when they plan to be on Spotify again."