Why did Americas founders adopt constitutional democracy as the U.S. Political system?

Having seen the economic and political problems associated with divine or absolute rule (and hereditary aristocracy), they wanted to avoid these impediments. The rule "by the people" was not a new idea, but most of Europe still retained their monarchies, and these often came into conflict with the wishes of their people.

It was also a logical extension to retain the Parliamentary form, and simply omit the king.