The way I understand it, the spelling of English words (unlike the French, which has had an academy deciding these matters for centuries) has never been high on the agenda. I was reading a biography on Shakespeare a couple a years ago and I learned that, at the time (4 centuries ago) there was no such thing as 'spelling'. Individuals wrote how they thought words ought to be written.
Move up a couple a centuries and not much had changed. Webster created his dictionary about 200 years ago (?) and his idea of the way spelling of words ought to be was based on writtings in the New World (in large parts).
Language evolves over time. By the time the Americans fought for their independance, they were not interested in following the British way of writing. Their evolution had been diff'rent from the mother country's evolution. And the numbers of Americans, even 200 years ago, allowed them to assert themselves in the spelling of words that reflected the commonly accepted ways in the former colonies. Thus, two seperate spelling ways for two seperate cultures sharing the same spoken language.
Now in Canada, we continues to be part of the British Empire long after the second word war (the Queen is still the head of this country!), so we learn and write British English.
As for us Canadian Fracophones, being shut out of the French world for nearly 200 years, we too had evolved diff'rently than the European French. So in the past 50 years or so, we've had to relearn the 'correct' words and ways of speaking and spelling en français, cause a the aformentioned 'Académie française'. However, recentry (about 4 years ago), a very comprehensive French dictionary for French Canadians has become much more useful for us 7 or 8 million francophones. Dictionary's called the 'Multi-dictionaire' and it's a lot easier to understand than the 'Larouse' or 'Le petit Robert'. It even lets us know if the word is old french, 'joual', or an actual Académie' accepted word.
Too much information? Sorry. Teachers are known to be long winded.