Why is MF ban/pick in bronze/iron?

Her Q.

Like, sure, it's partially her passive and R, but it's mostly her Q and lethality. High burst damage on an ADC and its fairly reliable, especially considering how often people still stand behind minions anyways.

I hate MF due to her ult. Its a 1v9 potential if teams group up in a team fight.

She's not only really easy to pick up and use, but she's also a big lane bully - Bronze/Iron can't really deal with those.

Because the game changes from Ranked summmoners rift to Ranked ARAM at around 12 mins of the game.

I ban MF and Draven even in Silver. MF is insanely reliable, easy to support, steady power spikes with a not too bad early game. Burst, minor CC, and that ult make her an absolute headache to fight.