Why is Spell Vamp disappearing?

Over the last few years spell vamp has gone from rare to practically nonexistent. And now with the new mid-season update info Riot has given us, Will of the Ancients is going away completely, and Hextech Revolver no longer gives spell vamp. I think this is the exact wrong direction to be going, as Riot is removing just about all the spell vamp from the game, while they keep giving more lifesteal items to the adcs. I really don't understand why they would do this.