From the perspective of productivity development, constitutional monarchy is the product of mutual compromise between the bourgeoisie and the feudal class when capitalism is not fully developed and the bourgeoisie is not strong. Therefore, countries with earlier bourgeois revolution adopted this state system to a greater or lesser extent.
By the time of the American bourgeois revolution, the strength of the American bourgeoisie had grown, with a unified market and a unified nation, and the productive forces of capitalism had also developed. In a revolutionary country under such conditions, the revolution is generally more thorough, unlike the constitutional monarchy with feudal remnants, so the democratic and harmonious system is adopted.