1 British made strategical mistakes
British troops were confident and underestimated the patriots. Also, General William Howe was presumptuous that he is likely to become the boss of The Bunker Hill battle. The lack of respect and the naive army resulted in losing the war against patriots.
2 Patriots were motivated and benefitted with leadership qualities
Even though the British troops had better firearms than patriots, patriots were still not hesitant to fight British troops. Some colonists did not want to fight and wanted to remain the part of the empire. Whatever they lacked in, all was made up for by leadership.
The troops added more people and made them believe in themselves. The other leaders also helped them overcome the odds and harness their men to fight. Thus, the leaders' proficiency was one of the significant factors that helped patriots win the revolutionary war.
3 British misunderstood the political nature of the conflict
British troops were bewildered in determining the nature of the war. They thought that it is a traditional war, but in reality, it was a revolutionary war. The British were fighting to become the king, whereas the Patriots were fighting for their freedom. It was one of the reasons why patriots were highly motivated when they participated in this war and made the best of their efforts to win it. (See How Did the Americans Win the Revolutionary War?)
4 France helped Patriots
France was secretly providing valuable weapons to the patriots. One of the purposes of helping patriots was that French and British always used to consider each other lifetime enemies. French did not want to leave this opportunity by assisting patriots in and making the British lost the battle.
Marquis de Lafayette helped George Washington and The Continental Army to acquire crucial tactics and provide his valuable knowledge facilitating them to win the war. (See How War Affects Economy?)