Because he can't win Ohio without white small towns going for him.
In 2004, out of 88 counties in Ohio, Kerry only won seven or eight, and those included cities like Columbus, Cleveland, Dayton and Toledo. Rural Ohio decides for the state. There are 11.5 million people in the state, and fewer 35% live in the big "Three C" cities.
You might want to add many small counties in SE Ohio voted heavily for Strickland and other Democratic state candidates in 2006, plus local politics lean to rat control. Given a chronically poor economy in the region, SE Ohio is not a sure thing for McCain, at least in the numbers needed to offset metro Ohio support for Obama.