because unlike the ultimates youve stated, mass rez didnt require LoS, made Mercy invulnerable, was instant and made it so Mercy would get POTG nearly 100% of the time.
I agree with the idea of the devs toying around with some changes / nerfs to Mass Rez before removing it entirely, but what was done is done. Nothing me or you can do about it, sadly, other than the fact that we get to experience it once more in OW2 