Why were the colonists able to achieve independence from Britain?

NEW ANSWER:

The secret that won the war for America was distance. A huge ocean separated them from Great Britain. The British had to ship arms and armies over this wide stretch of water, which was a very expensive undertaking. In the end this distance, as much as anything else, defeated the British.

Distance was an important factor within the colonies, too. American generals could retreat over vast, sparsely settled landscape without yielding anything of importance. British armies marched and conquered and found themselves controlling nothing but the soil beneath their feet, while a hostile wilderness closed in behind them.