Why did general Washington advance on Yorktown instead of New York?

He attacked at Yorktown because the attack was confidential and the plan was to take the British by surprise. Washington had originally thought of attacking NY but Lafayette had cornered the British in Yorktown and the French troops were on their way to that area to offer assistance.

He learned that the second French fleet would land much farther south ... The second French fleet prevented General Cornwallis from escaping from Yorktown by sea