Final answer:
The British lost the battle of Yorktown largely due to the French naval victory which trapped Cornwallis's forces with no reinforcements or escape route, ultimately leading to his surrender.
Step-by-step explanation:
One significant reason why the British lost the battle of Yorktown was due to French naval superiority. The French fleet under the command of Admiral de Grasse sailed into the Chesapeake Bay, securing the area by defeating the British fleet. This victory at sea effectively blocked any British naval support and escape routes for Lord Cornwallis's army, leaving them besieged on land by a combined force of American and French troops.
The British forces at Yorktown were trapped, as General Washington and the continental troops, along with their French allies, laid siege from September 28 to October 19, 1781. Cornwallis waited for reinforcements from Lieutenant General Henry Clinton that never arrived in time, leaving him with no choice but to surrender after his final breakout attempt failed when trying to move his troops across the York River to Gloucester Point.