Toward Ecosophy

I have a question about timing and astrology. The question of how to apply electional astrology or planetary hours/days comes up pretty frequently on Magic Mondays. The time a new business is established, for example, is the key moment for one common class of such questions. Why, then, when it comes to applying astrology to the founding of the United States do folks use our letter of resignation (Declaration of Independence, i.e., the infamous Sibly chart) instead of when we established the Constitution? Was this something specifically debated by practitioners at one point, or did folks just gravitate toward 7/4/1776 by default?

Thanks!