Because warlock have the best hero power in the game by a large margin. Blizzard have always had to be very careful about the power level of all warlock class cards because by default warlock's hero power puts them at an immediate advantage over any other opponent from another class.
Another potential reason is that healing is unusually good in warlock because so many of their cards (and also their relatively very powerful hero power) are balanced against HP loss. Many pros and streamers have said that whenever warlock gets strong healing tools, they dominate the meta (or some have said in the past that if warlock gets strong healing in future, they will almost certainly be a top meta contender). We can see this has been arguably proven correct with the reign of cubelock while Kobolds and Catacombs was in standard: Blizzard eventually nerfed Dark Pact from 8 healing to 4 healing.