Americans had many intangible advantages.
The British fought a war far from home. Military orders, troops, and supplies sometimes took months to reach their destinations. The British had an extremely difficult objective. They had to persuade the Americans to give up their claims of independence. As long as the war continued, the colonists' claim continued to gain validity. The geographic vastness of the colonies proved a hindrance to the British effort. Despite occupying every major city, the British remained as at a disadvantage.
Americans had a grand cause: fighting for their rights, their independence and their liberty. This cause is much more just than waging a war to deny independence. American military and political leaders were inexperienced, but proved surprisingly competent.
The war was expensive and the British population debated its necessity. In Parliament, there were many American sympathizers. Finally, the alliance with the French gave Americans courage and a tangible threat that tipped the scales in America's favor.