The answer, sadly, is largely racial. In the late 19th and early 20th centuries, the U.S. never built up a labor movement, let alone a labor or socialist party, with the power of those in most European states because racial animus prevented the black and white lower classes from organizing together. It was hardly the only factor, but it was a critical one. To this day, race is the best predictor of support for welfare, and during last year's debate, racial animus was correlated with opposition to health-care reform.
Sadly, this country's horrid history of racism will never fade entirely away.