There are many reasons why the colonies were able to defeat Great Britain. Three of the most important ones are:
Help from France. It is almost inconceivable that the Americans could have won without help from Britain’s rival superpower. French money, ground forces, and naval forces all played huge roles in the colonists' victory.
Better motivation. The colonists had much more at stake in the war than did the British. This was true both for those fighting and for their political leadership.
An easier path to victory. For the British to win, they would have needed to change the minds of the colonists and make them happy to be British. This is hard to accomplish militarily. By contrast, the colonists only had to keep from losing long enough for Britain to tire of the war.