A good question, and one that’s asked a lot! Isn’t the English language difficult enough without having different versions?
Well, the answer comes in different versions too but there are two main reasons.
Firstly, the natural divergence that occurs over hundreds of years between two countries thousands of miles apart. I am, of course, talking about a period before aeroplanes, television and computers.
Secondly, Noah Webster, of Webster’s Dictionary fame believed it was important for America, a new and revolutionary nation, to assert its cultural independence from Britain through language. He wrote the first American spelling, grammar, and reading schoolbooks and the first American dictionary. He was also an ardent advocate of spelling reform and thought words should be spelled more like they sound. Hence, reasons Americans spell words such as “favor” without a “u”, “theater” with an “-er” instead of an “-re” at the end, “sulfur” with an “f” and not a “ph” in the middle, and “aluminium” as “aluminum.”