Why was the letter 'U' dropped from many words such as colour, odour etc? And also why is the letter 'l' spelt as a singular, when in British English it's used as a double 'll' in most words, cancelled, traveller, modelled etc?
Many of the differences between American and British English date back to a time when spelling standards had not yet developed. For instance, some spellings seen as "American" today were once commonly used in Britain and some spellings seen as "British" were once commonly used in the United States. A "British standard" began to emerge following the 1755 publication of Samuel Johnson's A Dictionary of the English Language, and an "American standard" started following the work of Noah Webster and in particular his An American Dictionary of the English Language, first published in 1828.
In A Companion to the American Revolution (2008), John Algeo notes: "it is often assumed that characteristically American spellings were invented by Noah Webster. He was very influential in popularizing certain spellings in America, but he did not originate them. Rather […] he chose already existing options such as center, color and check for the simplicity, analogy or etymology". William Shakespeare's first folios, for example, used spellings like center and color as much as centre and colour.