Yorktown is on a peninsula, facing the York River. The Continental military did not have the naval power to block the British Navy from entering Chesapeake Bay and providing support, ie. bombardment of US positions, and reinforcements, or if necessary, evacuation of British troops.
The only way that Washington and his generals could trap the British Army and force the surrender was through the help of the French Navy, who DID prevent the British Navy from entering the Bay and providing that support. French land troops bolstered the Continental Army as well, but it was the Naval support that truly turned the tide and made surrender the only option for Lord Cornwallis and his British
