There are a number of theories that try to explain the reason Brits and Americans speak English so differently.
As the American colonies gained their independence from Great Britain and became the US, they grew and integrated into different cultures. So, Americans and the Brits developed linguistic differences. General phrases that already existed in British English changed. For example, minced beef became ground beef.
On the other hand, a theory blames Noah Webster who, following independence, created the Webster Dictionaryto reform the language in America.
With the help of Expert Market's video, on "British vs. American Terminology," it shows just how different the same language can sound with just a matter of distance.