The Preamble includes the phrase "to form a more perfect union." Why is this reason listed for creating a new Constitution?

The phrase "to form a more perfect Union" has been construed as referring to the shift to the Constitution from the Articles of Confederation. Later, the phrase came to mean the continual process of improvement of the country.

The Preamble states that the Constitution exists "to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, [and] promote the general Welfare." The emphasis on establishing a "Union" and a successful government for it is not surprising. The phrase "to form a more perfect Union" has been construed as referring to the shift to the Constitution from the Articles of Confederation. Later, the phrase came to mean the continual process of improvement of the country. It declares that what the people have ordained and established is "this Constitution" referring, obviously enough, to the written document that the Preamble introduces. That language is repeated in the Supremacy Clause of Article VI, which declares that "this Constitution" shall be the supreme law for the entire nation. The written nature of the Constitution as a single binding text matters and was important to the framing generation.