it’s quite simple really, the reason is purely geographical. Language is a living thing, and evolves, just like the people who speak it. So if these people are all locked up in a given space (whether that is a small pacific island or the US), the language will evolve there. The bigger the population, the faster the evolution. Which is why the fairly isolated Lithuanians currently speak Europe’s oldest living language (the closest to Sanskrit) whereas the Americans changed from British to American english (even the spelling) in just 200 years. Similarly, Australian English is slightly closer to British English because the population that speaks it is smaller. Note that in both these countries the language has been affected also by external influences, such as Italian immigrants, native populations, etc.