explain why American's didn't embrace the revoluntionary social theories purported by Marx and Engels?

because Americans value freedom (or at least did). I agree, charity and giving a hand up is very virtuous... however it ceases to be when forced by government and the barrel of a gun.

It was thought then (as often now) their theories which were basically social and economic...

1. Did not allow enough for (or allow at all) political democracy.

2. That if implemented they would wipe out the rich, the elite, and business owners which was seen as not desirable and who were way too numerous and had way too much power in the USA to every want to change.

3. That their theories were anti-religious and denied God and religious freedom.

You mean besides being antithetical to the principles of the Constitution? And how is it socially "fair" to bust your a** for 10 years to become a Dr just so you can live in exactly the same lifestyle as a ditch digger? Saying that both of them should have exactly the same social worth and relevance is delusional.

America was founded to be as far away from Communism and Socialism as possible.

Because America is, or used to be, about achievement and what you call social fairness is mediocrity and Communism.