Why is the US a limited government?

In simplest terms, the Founding Fathers were convinced by the old saying 'Power Corrupts'. To their minds, concentrations of power produce infringements on rights and liberties, sooner or later, and so they thought the best system was one which distributed power over as many people and structures as possible. the limitations on the individual branches of government, and on the government as a whole, were designed to make tyrannical rule next to impossible.