Because the Mass Rez ult encouraged toxic behavior where the Mercy would hide then yell at everyone to go die on point so she could fly in and get the huge rez, and gameplay became oriented around people dying on point then being rez’d, which Blizzard didn’t think was good. It wasn’t just that the Mercy was doing hide-n-seek or that the enemy team had trouble countering it (though I’m sure those contributed), but that the behavior with the Ult spilled over into the entire team and, moreover, the entire match