>Why the US doesn't have a strong Left or social-democratic tradition?
Well we sorta had one once but it's gone. Short answer: immigration in waves led to self-definition by ethnicity rather than class; slavery led to a self-definition by race rather than class; immigration self-selects a bunch of strivers; vicious capitalist class with a retrograde ideology and strong taste for force; purges, raids, repression (including private repression - blacklisting etc); culture of individualism, self-improvement, self-reliance, and competition; role of highly masculinized imperialism in creating a culture and practice of violence; periodic waves of orchestrated panics (Germans, Russians, Muslims); etc.
I'm sure others could add many more.
Doug