In football it is called the home field advantage. Morale is boosted because you are playing in the midst of your own fans. There is a shortened supply line and the locals are generally friendly to...
The British were fighting for money and pride. The Americans were fighting for their freedom and homes. Sure, we ran a lot and got the French to fight, but that's part of our charm. At least back...
They were fighting on their home turf so supplies and instructions didn't have to travel 3000 miles like it did for the British. They had support from the French (ahs)