Hi and thanks for your suggestion!
Spotify wants all the world`s music there, however adding music on Spotify is almost always a decision by the artists or their labels!
There can be several reasons why artists or songs are not currently available on Spotify. Sometimes agreements can’t be reached with the artist or label or a change may happen in music ownership.
For more info: Content missing | Music FAQ's
If you want more music in Spotify, you can fill out this form for Music Request! : )
You might also be interested in this idea which suggesting simpler method for requesting music! :