The problem was CAFE; the Ranger was too small and too inefficient to meet increasingly stringent CAFE standards. CAFE rules are based on the "footprint" of the vehicle, basically the area between the wheels, and in 2012 the "light truck" target would edge past the Ranger's footprint/economy combination. There's a detailed article here: http://www.thetruthaboutcars.com/2012/10/how-cafe-killed-com...

Redesigning the Ranger to have a larger footprint results in a mid-sized pickup only slightly smaller than the base model F-150, which is where the sales-cannibalism problem comes in.