Lack of a clear strategy. The British initially sought to suppress the rebellion, but they lacked a clear plan for how to do so. They also underestimated the strength and determination of the American colonists.
Lack of support from the American people. Many Americans were initially loyal to the British Crown, but as the war dragged on, public opinion turned against the British. This was due to a number of factors, including the British use of force against American civilians and the passage of the Coercive Acts.
The French alliance. France entered the war on the side of the Americans in 1778. This gave the Americans a much-needed ally, and it also helped to drain the British treasury.
The war of attrition. The Revolutionary War was a long and bloody conflict, and it took a heavy toll on both sides. The British were eventually unable to sustain their losses, and they were forced to surrender in 1783.
The Revolutionary War was a complex conflict, and there are many other factors that contributed to the British defeat. However, the factors listed above were some of the most important.