Question

Why do you think Dr. King (Martin Luther) refers to the Constitution and the Declaration of Independence in his speech?

Answer 1

It talks about all men are free.

Answer 2

I think Dr. King refers to the Constitution and the Declaration of Independence in his speech to show that everyone should be free and treated equally because the Constitution and the Declaration of Independence have things written in them that say, for example, "All men are created equal" to show that everyone should be treated equally.
Hope that helps you.