Because "more healing" is not a utility that is needed in this game. Does no one see that? Why do you think Monks are starting to be benched now for the meta comp? Traited Mantra is actually an insanely good cooldown, except for the fact that this game has no healing requirements that make use of it.

I'm in favor of changing Balance so that it's something that doesn't boost DPS, but changing it to something that boosts healing when healers already have a bevy of cooldowns that work just fine on their own won't really make it a desirable card, it'll just get Royal Roaded for the enhanced effect and used to power up an Arrow for more DPS or a Bole so that the tank can either drop tank stance and do more DPS or so the healers can heal less and...do more DPS.

I know I tend to drag everything back to the "DPS meta" sometimes, and I'm sorry if I sound like a broken record, but it's frustrating because everything being pared down to "do your primary role as little as possible so you can DPS MOAR" is why cards are in a crap situation like this to begin with. If extra healing made fights way easier, but not required to clear? It might become more valuable. If that damage reduction helped ease mana consumption, save precious mitigation cooldowns and lower the healing load instead of giving tanks/heals more opportunities to DPS? It might become more valuable. THIS is why I'm so against the meta, because everyone's tools get watered down like this and there becomes only one real choice - the choice that better boosts group DPS. In a situation where AST cards have multiple functionalities and all those functionalities are viable because mitigation requirements and healing requirements are stricter, all the cards would start to see a lot more use, especially given certain situations. Yes, when healing or mitigation isn't readily handy, AOE Arrow would be the go-to, but it's such a marginal group DPS increase as it is that using it once or twice a fight wouldn't break any meta except at the world-first level. But you might start to see stuff like, "this hits hard, so Bole is more valuable here" or "these moves hit back-to-back so increased healing on everyone is more valuable here", or at best the efficacies would be the same so a healing up card and Bole would at worst become interchangeable in those scenarios.

Just so I don't sound like I'm only posting to preach from my soapbox, I think a change that could be done without drastically altering healing design in this game would be to have Balance operate like a mini Battle Litany instead of the flat damage boost we have now. If it has around the same efficacy as Arrow, AOEing the two would probably become a more interchangeable outcome and it would make the other cards feel a little less "dud"-ish in comparison. I've seen that suggestion bandied about here before, but can't remember what the healer community response to it was.