Noah Webster’s Influence → After independence, Noah Webster promoted spelling reforms to distinguish American English from British English. His American Dictionary of the English Language (1828) standardized many differences, such as:
Colour → Color
Centre → Center
Defence → Defense
Isolation from Britain → With political separation, linguistic changes in Britain (such as Received Pronunciation) did not affect Americans as much.
Influence of Other Languages → American English absorbed words from Native American languages (raccoon, moccasin, squash), Spanish (ranch, canyon, tornado), Dutch (cookie, boss, sleigh), and others.
Westward Expansion → The movement across North America introduced new vocabulary related to geography, wildlife, and culture.