Well they also systemically have been removing all forms of DoT from the game. I would imagine that has something to do with it.
Viktors laser used to have a DoT on it and they removed it for the after shock effect. Cassiopeia used to do more poison DoT damage and they changed her into a Twin Fang machine gun. Swain was a DoT champion and they changed him into a catch and burst at melee range champion.
At the moment who is left? Malzahar might be one of the only ones still around.
I miss the DoT's, it's one of my favorite play styles, but it hasn't been well supported.