The continental army had strong military leadership like George Washington and The Marquis de Lafayette. George Washington had many smart military decisions that led to many victories.
The colonies allied with France, an enemy of Great Britian. The French provided the colonies with troops, financial support, and weapons.
As the war dragged on, the British grew tired. They were not fighting for any cause, unlike the colonists who were fighting for their independence.
Great Britain had a strong army and navy. However, the Continental Army lacked a strong military. They were untrained and made up of soldiers from each of the thirteen colonies.
The Continental Army lacked supplies and troops. For example, at Vally Forge, due to the shortage of clothing, shoes, and food, the cold winter killed about 2,500 of the troops.
The Continental Army was not a professional army like the British Army was, and they clearly did not have the same number of troops either. The British army was made up of 80,000 troops. The British had the money to pay mercenaries to fight for them, while the colonists did not.