Availability of music on Spotify is up to the artists (or representatives / rights owner) and / or their labels. Music missing can be a decision of one of them.
Music missing can also be related to a licensing problem. Licensing rules are not the same from a country to another. That can lead to have music available in some countries and not in some others. Usually when a licensing problem is solved the music can be added or come back if it has been removed.
Spotify wants all the music in the world to stream and works hard daily for trying to convince artists and labels to let all their catalog in streaming.