Dave Wilton of Wordorigins.org has done an admirable job of sorting out the history of main street (primarily US) and high street (primarily UK) in this post. His conclusion:
More likely the divergence between the British High Street and the North American Main Street is simply due to the opening of a lexical niche with the creation of cities and streets in the New World and the decline in the use of high to mean “principal.” The streets on the new continent had to be named something, and Main just happened to gain a beachhead, perhaps because at the time the meaning of high was being narrowed, and its use as a descriptor seemed more enigmatic. Once established in Boston and a few other cities on the eastern seaboard, Main Street was then carried westward to new towns.
But by all means read his whole post and enjoy the details.