There are 3* troops with hp regeneration bonus. But 4* troops are critical hit chance bonus and mana regeneration bonus. Why not create 4* troop with hp regeneration bonus?
Update: hp regeneration meant healing bonus. And troop as 4* with additional hp with healing bonus.
Oh yes. My bad. Sorry guys, I meant healing bonus as hp regeneration. Little not noticed healing bonus in mana troop. But there are no troop with additional hp and healing bonus then.