Healers are problematic anyway just because of having to target something other than opponents. Think about it: If an enemy attacks a player in this game, the enemy is usually auto targeted so there's no need to think about having to move and click the target or tab to it. For healers, there is no equivalent for players. If someone is getting hit by a monster, the fastest way to swap to the player is sometimes to target the monster and then do target of target. Then you have to pick out what healing skill to use, which typically means prioritizing OGCD heals first before those with mana cost and/or cast times. That's before including the decision to use something that cuts MP costs down or allows instant casting like swift cast.

Cutting back on the total needed healing skill bloat and giving healers a similar design to tanks (only for healing) probably would help with the popularity, but I think the player targeting aspect would make them more complicated to run than a Tank under any kind of situation.