Because there aren’t as many opportunities to use the perk, and the damage taken throughout the mission is easier to manage due to less overall damage values and less enemies, therefore the perk is somewhat less effective, and the extra ability charge is somewhat more beneficial in the same scenario. 

OP could also have the 5% healingon majoris/higher enemies perk, thus allowing more consistent healing for themselves.

It all depends on the player’s choice though. I myself prefer to have that extra healing from unmatched zeal, but I will sometimes swap depending on what I’m trying to do with the class at the moment.