Because on paper everything about Bastion is an advantage for his enemy, and a disadvantage for himself.
His only advantage is supposed to be increased damage, as in, VERY high damage and very punishing damage, when he’s not able to move.
Is it punishing, or high damage in practice?
It’s tickle damage when your enemy even let you hit them, and even if you get the drop on them, you have to have like a full 10 seconds to kill any meaningful amount of the enemy.
Bastion cannot deal headshot damage, and thus is restricted to sometimes being almost barely playable when an entire team enable him, and even THEN people act like a Bastion securing kills, is somehow bad.