I can't remember the reasons, but I read an article some time ago about Spotify. They said about 4 million songs (20%) on their servers has never been played.

There is even a third party app created that randomly suggests never listened to songs based on your criteria.

http://forgotify.com/

It's tied into Spotifies discovery technology. Spotify do want you to expand your listening and find new music as it maintains the interest for musicians to keep utilising the platform.