Here's a tidbit. American English is less modified from the older English than British English. That means the extra U in so many British spellings is the variation from formerly proper English while American English retained most of the original spellings.
By comparing the former and current English practices, the extra U is a British invention, not originally an English standard. American English could be considered more pure - or just archaic.
TL;DR - Americans didn't drop the U in words like humor or color. The British just added one.