Because ledge dash across map was considered OP by the devs so they removed it, but superjump was a fair interaction and they liked it, so it was left in. They said this in their AMA.
If game mechanic exploit and/or bug creates unfair interactions (such as ball infinite boop killing tanks in microseconds of connecting, junkrat droptire making riptire invisible, placing symm teleports and turrets inside walls), it’s removed. Doesn’t matter if some players feel like it’s gameplay enhancing to learn to do stuff like that.
Superjump hasn’t necessiated developer time in tuning because you’re not jumping too high and superjump has all the same guardian angel limitations as normal ga. And it’s not like mercy can’t access high ground or jump high from players without other maneuvers, they just require more teammate micromanaging than just letting mercy have slightly bit more agency on her own.
Devs have taken time to tune some gameplay mechanics before despite liking something because they felt it was too strong, like doomfist rollout distances. But they’ve intentionally left something like diagonal punches in.