Old Style: Why Did THIS Brand Become Associated with Chicago?

I'm from Milwaukee and the beer of choice there is/was Miller Lite because Miller is/was headquartered in Milwaukee. When I moved to Chicago it seemed as though every block had a tavern with an Old Style sign above its name so I asked myself "why is that Chicago's beer of choice?"
It's not made in Chicago.
It's not bottled in Chicago.
It's not particularly good.
In this Chicago Bob Short, Shrog and Bob delve into this mystery.