The United States Constitution is an amazing document. It is the oldest single-document constitution in the world today...pretty amazing for a country as young as ours.
It was penned by the Framers in 1787 and created a national government that was supreme to any and all state governments.
This national strength created a fear by citizens that an unchecked central government could, and probably would, trample on individual rights.
To assuage those fears and assure ratification, the Bill of Rights was added to the constitution in 1791.