Answer:
Main reasons were good strategy of Continental Army and help that France provided to Patriots during the war.
Step-by-step explanation:
When the war started Britain had its soldiers located in the colonies, that were later supported by new troops that arrived. Still, we need to have in mind that it was practically impossible to provide new troops, resources, provisions in short notice, as they needed to come from Europe.
On the other side, it is important to add that after the battle of Saratoga, France started supporting Patriots with army, supplies, even with money.