The correct answer to this open question is the following.
The reason that Cornwallis gave for staying at Yorktown and trying to defend his position there was that he had received orders from General Clinton -the commander of the British troops in the Revolutionary War- to protect the harbor in the Chesapeake Bay, to guard the British fleet. Lord Cornwallis chose Yorktown because it had deep waters in the York River. But by that time, the Continental Army led by General George Washington had already received the support of 5000 French soldiers that helped the Americans to defeat the British.
