Partly 4 July 1776 is treated as the start date for the US because that's the date everyone thinks of -- the popular imagination is a fairly good indication, since it's shaped so powerfully by subconscious factors. Partly, though, it's because progressions and transits based on that date have yielded better results than those based on other dates. It wasn't debated as such -- it's just that each generation of astrologers assesses the options and chooses which one to go with.