1) The colonies were too large and spread out. They lacked a capital for quick and easy capture, which made the colonists impossible to completely control.
2) Washington made a point to avoid confrontation, which also made it difficult for Britain to take control of the conflict.
3) America was receiving help from the likes of France, Spain, and the Netherlands.
4) Britain was spinning too many plates, so to speak, and they couldn't afford to continue fighting for this colony as other colonies were falling apart in the West Indies.
5) The British didn't trust Loyalists enough to allow them to fight for the Crown.
6) Americans knew more advanced military tactics than the British, including certain forms of guerrilla warfare learned during conflicts with Native Americans.
