Simple answer to all dialectal variance, isolation and distance over time. As the two are isolated from one another, each inherits and purges different sounds and words according to internal and external influence (population migration, immigration, new discoveries, phrases etc).
All languages are living breathing entities that evolve over time. If you take one, and split its population (like AmE and BrE) over time they are going to sound and be implemented differently. It’s the same reason the French and Spanish etc don’t speak Latin anymore, after the fall of the empire they were isolated and eventually grew into the languages they speak today.
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