It's not written anywhere how they decide on what Pokemon got banned but they test them. A Pokemon might have a high attack stat but have other stats sub-par.
Look at Moltres and Mewtwo for example, they are both legendary but Mewtwo is banned while Moltres isn't.
Mewtwo's BST(Base Stat Total) is higher by 100, standing at 680, while Moltres' BST is only 580.
Moltres does have 100 base attack and 125 base special attack but it's slower and has lower defense stats than a lot of Pokemon used in higher tiers, outclassing Moltres easily.
That being said, the BST might make the Pokemon appear weaker than it actually is.
So the answer to your question would probably be the BST mostly decides what tier a Pokemon is going to be, alongside with its overall performance.