The power of government comes from the people.
This concept is known as the "consent of the governed." This idea, implemented in the preamble, was an Enlightenment idea discussed by philosophers like John Locke. In his book Two Treaties of Government Locke discusses how the government derives its power from the people, as they should have the ability to elect officials into office who will best serve their needs. This is why the Founding Fathers implemented this idea into the preamble.