Have you ever wondered why the spelling of some American English and British English words are different? It has to be taken in the context that 13 major colonies, which formed the backbone in the formation of the United States were once under British rule between 1607 and 1732.
The United States was declared independent from the British Empire on July 4, 1776 after they won the American Revolution. It was a hard-fought independence, which went on from 1765 to 1783.
Therefore, it goes back to the question as to why America and Britain spell some English words differently when the former was under the latter's rule for about 125 years.
Noah Webster, the creator of the Webster's dictionary
Noah Webster believed that as a new nation, the United States should affirm its cultural independence from the country that colonized it via language. It is a way to distinguish an American from a British person. Webster started the first spelling, grammar and reading books for schools in what would later be termed American English while believing that these words should be spelled as they sound. He was also the creator of the first dictionary in American English.
Webster first created a small yet quite revolutionary dictionary called the Compendious Dictionary. In it were words that were drastically changed in the final version, the American Dictionary of the English Language. He started it in 1807, with the two-volume dictionary published in 1828. Ironically it sold 3,000 copies in Great Britain, while only 2,500 were bought by the Americans. It was attacked for its unconventional use and spelling of many words and the inclusion of other non-literary words. In short it was attacked for its "Americanism." In it were found the new spelling of words such as aluminum instead of aluminium, sulfur rather than sulphur, favor without the letter U and theater rather than theatre.
Webster introduced a set of spelling that distinctively stamped them as American without changing the words too much so they could still be mutually understood. Likewise he added some systematic changes such as using –ize rather the –ise for Greek-derived verbs.
Formal versus less formal
If this is to be taken as the second reason for the difference in spelling, one of the best examples is the word "whilst." In the United States, the word "while" is used instead of whilst and although their meaning is the same (period of indeterminate length; at the same time that; or even though) it is believed that whilst is used formally, according to World Wide Words. Therefore it could be surmised that whilst fell out of favor because Americans are less formal than Britons.
Simplification and economics
Before the publication of the Webster dictionary, it was acceptable in the UK and the US to use either form of spelling in several words such as fibre/fiber, defence/defense or humour/humor.
Webster was quite influential and became an authority in the United States, and the changes he had made spread to England as well, including the dropping of the letter K in words such as publick and musick. Some of the changes remained in use in the UK although there are some that still continue to use other spelling variants, such as words ending in –ise rather than –ize although the latter is considered technically correct in the United Kingdom.
Presently, economics has some influence in the use of words. Modern British tabloids have adopted the use of shorter forms of words, removing the extra letters that otherwise do not change the pronunciation and meaning of the words, such as the U in favour, labour or honour and program, instead of programme. Of course, serious newspapers in Britain are not quick to adapt to the modern standards.