How did British English and American English get so different?

Recall that the United States was once a British colony in the same way Canada was. Yet somehow British English and American English somehow became so different. The words for things are different, the pronunciation is different.<br><br>How did this come about so quickly?<br><br>Edit: Quick note: This is not aimed to be antagonistic towards anyone. If anything it’s out of curiosity: I’m British and my wife is American. When we make food my wife always makes sure to differentiate the two types of beans by calling the American ones “bush beans”. I just find it all fascinating.<br><br>Also: I understand that it’s not a case of one being superior to the other. That goes against the spirit of history.