Question

Why was Yorktown seemingly a good place for Cornwallis to establish fortifications

Answer 1

Cornwallis wanted to be connected with other British troops and naval forces to get supplies.

Answer 2

Yorktown is a town, and really strategic point (on war times) located in Virginia, near to the York River.

Having control and fortifications in York near the York River, where the french allies and American resistance were navigating, they thought that starting their defence,since the first moment, when the invaders were still in their fleet, was an important advantage (some weapons used by Cornwallis army, were specialized to damage ships and other warfare structures).

So based on this (start to attack before the enemy) and the fact that Cornwallis mistakenly thought that around 8.000 man was enough to repel the adversary attack, made him think that Yorktown was the place to be, on this difficult battle.