Same reason you don't see major sponsorships from non-automotive companies... not enough return on investment. Since rallying is little known in the U.S. (though it is growing again), Ford is not about to dump a whole load of money unless it translates to sales. So they spend their motorsports budget on NASCAR, CART, drag racing and off-road where there are a lot of U.S. fans.
That said... look for a major ProRally effort involving a Ford product during 2003.