Question

Why did each of the 13 states write a constitution soon after independence was declared?

Answer 1

They knew they needed to replace the British government after the war was over, and the Continental Congress suggested that each state write its own constitution.

Answer 2

After the American War of Independence had begun, the Congress asked the  13 original colonies to form governments for themselves, so the colonies began writing and adopting new constitutions to become sovereign and independent states. By 1780, every state had a written constitution.  Under the Articles of Confederation, the power of the national government was centered in the Congress. However, the national government was weak and states functioned like independent countries. So, at the 1787 Constitutional Convention, delegates from the different states decided to a plan a stronger federal government with three branches (executive, legislative and judicial) all of equal power. As a result, the US Constitution was signed on September 17, 1787 in Philadelphia.