re: Why do cities favor leftist politicians?

Historically, and I mean possibly to Sumer, cities are the areas where cultures are made and develop. This increases the progressivism and liberalism of society, generally. They're also the economic drivers.

Now, do they push the envelope? Yes. Too far? Yes. Which leads to a cultural, conservative societal response. This ebb and flow continues on and on with a net left effect until the society collapses (for whatever reason, which sometimes is decay from that liberalism).