Answer: Because of his pivotal role in the document's drafting as well as its ratification.
Explanation: James Madison is known as the Father of the Constitution because of his pivotal role in the document's drafting as well as its ratification. Madison also drafted the first 10 amendments -- the Bill of Rights. ... Ten of these amendments were ratified by the states and have been enshrined as the Bill of Rights.
He wrote and pushed the Bill of Rights through the First Congress and on to the states for ratification.
While no single person was alone responsible for the Constitution if anyone is entitled to the mantle "Father of the Constitution it is he.