Why does lincoln date the foundation of the nation from 1776 (the date of the declaration of independence) rather than 1787, when the constitution was written?

In 1863 President Abraham Lincoln stated at Gettysburg that the beginning of the new Republic was founded on the 4 of July in 1776 with the Declaration of Independence. He said that the Founding Fathers had written the Declaration of Independence creating a new nation where all men are created equal. Lincoln thought that the Declaration of Independence was a higher document than the Constitution written in 1787 or the Articles of Confederation because it was the first document and the one who expressed all of the American ideals.