Because balance =/= fun
Fun does not mean “overpowered”, by the way, don’t come with that argument.
Fun, however, is specialization. A reason to pick Mercy over any other support.
And Mercy’s specialization is… nothing. She’s too loaded. Can do too much, and will do all of her things below average, and is easily outclassed by other, specialized supports.
Mercy is supposed to be specialized in healing. At least, before Ana and Moira she was THE main healer that could heal the most. Right now she’s not even that, and is nerfed to be WORSE of a healer.
So, what then. Resurrect? That clunky ability, usually a suicide button? Meh, not worth it. Either the enemy gets 7 kills and an ult advantage. Or Mercy dies and it becomes a 6v5 at best, and is still a lost teamfight. The rare chance where a mid-fight rez is successful is too rare and too much of a liability to be worth it. Tempo rezzes alone are not worth it either.
Damage boost? Here, do a comparison:
Either you have a Zen with Discord that gives every ally a 30% damage bonus to a single enemy target (usually called out as focused target), AND can heal for 30hps at the same time
OR, you have a Mercy with damage boost that gives just 1 ally a 30% damage boost against every enemy, and disables her heals.
Mobility maybe? Mercy is pretty mobile.
Well, not as mobile compared to Lucio, who has freedom of movement.
Or as Moira, who can go invulnerable, AND has freedom of movement.
Survivability?
Heh, Moira can heal herself for 105 hps in the middle of combat.
Lucio can heal himself for 12.5 hps in the middle of combat.
Mercy can… not heal herself when fired at. Great.
So, literally got nothing going for her. “balance”, yet outclassed in everything she can do by others.