I suspect that it's a number of things. Not the least of which is that it's easiest to point to on a calendar. If one were to compare a nation to an individual, July 4th the day that the Declaration was ratified, could be treated as similar to a birthday. If one were to continue the analogy, the writing and ratification of the US Constitution could be seen as an individual who "gets their shit together and figures out what they're going to do with their life". It's a blurry comparison, because the US Constitution is the document that actually founds the Union and sets the structure of the government. But (trying to assist the analogy), a newborn can't walk, talk, or express its thoughts clearly to other humans (short of "I am hungry, I am uncomfortable, etc." and can only express those by crying). The birth is a landmark, but so is the transition into a self-aware, self-determining individual. But the latter is a process, and the former is an event. As I said, it's easy to point at the calendar and say "July 4, 1776 is the date when the Declaration of Independence was ratified". But the Constitution? A much more drawn out process with many stages and steps (much like the maturation of an individual). Delaware was the first state to ratify, on December 7, 1787 and the last was Rhode Island on May 29, 1790." Over 2 years before the 13 all signed off, and the last hold-out was essentially coerced and threatened with being treated as a foreign nation unless they ratified it. So, even though there is a Constitution Day that is designated by a single day on the calendar, the acceptance and ratification of the document took a long time and a lot of debate, argument and compromise (a number of states were arguing over whether the Bill of Rights should be in the main document, should be written as amendments, or whether or not they should be in the document at all).