Why do (awesome) songs dissapear from Spotify managed playlists?

[st3rv1s]

Hey guys, 

I am following a few managed playlist by Spotify and every now and then awesome tracks just dissapear for no reason. 

Now I can understand you don't want to a playlist to be too big... but who or what decides the adding/removing of tracks from these playslists. 

Of course I can make my own playlists... but I also like discovering new bands in playlists managed by others. 

[MattSuda]

Depending on the playlist, Spotify usually keeps the playlists updated with fresh content so it's not always the same.

[st3rv1s]

Then why not just build the playlist? Instead of removing great choices.