Spotify adds all the music they have the rights to, which means that if there's a song you want that isn't available on Spotify, it's because the artist/label hasn't given permission for Spotify to add the music. The best thing for you to do is contact the artists/labels directly and talk to them about why the music isn't available. You can also express that you'd like the songs added and help Spotify guage interest by filling out this form, but it doesn't guarantee the music will be added.
Hope that helps!
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