I don’t think this is the reason even though we’ve reached the same conclusion. She’s supposed to be the anti-flanker support, and that necessarily means that she has an easier time killing flankers than they have killing her. Otherwise, they have no reason to fear her and her niche is gone.
Considering how mobile Tracer and Genji are, that means that a Moira in the same skill bracket as a Tracer or Genji player needs to be able to consistently hit them, and moving around a lot as those two requires no higher skill than tracking a fairly wide beam against them with all the blinks and double jumps.