As stated before by Spotify:
"Many folks in the Community have requested certain tracks, albums or artists be added to Spotify. The reality is that we would like to have all the world's music available on Spotify.
Therefore, anything which we have the right to upload and has been provided to us, should be available on Spotify. We are signing new labels and working hard to add music every week. Hopefully your favourite music will be up soon.
If you still can't find the music you're looking for, try contacting the artist or record label and ask why the music isn't on Spotify.
You can also open this form and provide us with feedback. When doing so, do please keep in mind that this is an internal list we will use at regular intervals, to see how much interest there is for any given band. Submitting a name in that form or this thread doesn't mean they will magically be showing up soon.