The differences didn’t just come about due to an ocean being between the two countries, but because America is the “melting pot”. The American English language is just like the country, the United States. Our ancestors came here from many countries and brought their native languages with them. They spoke the languages and dialects from their countries and just as the people assimilated in this country so did many words from their languages. In the process, new pronunciations and meanings evolved and words from other cultures became part of the “English” language of the new country.
The United States isn’t just a melting pot of people who descended from different nationalities. The American English language isn’t the “King’s English”. It’s uniquely American-another language.