Question

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

Answer 1

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. ... As a result, the US Constitution was signed on September 17, 1787 in Philadelphia

Answer 2

After the American War of Independence had started, the Congress had asked the  original thirteen colonies to create governments for themselves, so the colonies s started writing and taking new constitutions to become all powerful and independent states. And to that, the US Constitution was signed on September 17, 1787 in Philadelphia.