I read an article once relating to this topic. It stated that back when America was rebelling against Britain, we wanted to find as many avenues to differentiate ourselves from the English as possible. One of the ways to establish our new identity as Americans was to alter the spelling of some words such as color/colour, rumor/rumour, governor/governour, etc. If I'm not mistaken, Webster was actually the one to formally initiate this change in spelling when he made his dictionary. Interesting stuff.