Why did most americans reject marxism in the early 1900s

They believed in a system that rewarded hard work with wealth.

The motto of United States is that it is a country of opportunities where everyone can prosper if they work hard and if they are smart enough, not that it is a country where you have no progress and you stagnate. This was deeply embedded in the mindset of the Americans who firmly believed that with hard work you can progress and get promotions, and run your own business, and get wealthy because they have deserved it, so Marxism came as a very repulsive ideology that was rejected immediately.

HOPE IT HELPED! :)