Answer :

They had the hame field advantage so to speak.  Also, they had the motivation.  For the British it was just another war, but to the colonists it was everything.  Lastly, they just had to fight to a draw and the British had to win the entire land back.

The Americans had a sense of Patriotism, (they were highly motivated to fight) and they knew the land they were fighting on. (North America)

Britian had to wait months for troops to arrive at the American colonies, whereas the colonies only had to wait days for more troops to arrive from the other colonies.