why do songs dissappear?

I was wondering why some music can't be played.

When I first started on Deezer there was also levellers song I like but it greyed out, I deletes it, thinking an error occurred, but searching for it, it couldn't find it.

Now I have several songs from Andre Reu which have greyed out. Can they be obtained again?

Why does this happen? 

I prefer they way Deezer is to Spotify, it is annoying, Deezer needs to be able to have the songs.