The intent of the award is to recognize a "leap" in the evolution of the genre a particular vehicle segment.
Silverado HD: Good upgrade, but essentially serves the same market and covers no new ground
Jeep Grand Cherokee: ditto
Dodge Durango: see above
Ford Explorer: Ford threw out the book on what Explorer was and re-wrote it to bring Explorer in line with where the marketplace is heading, all while paying heed to her existing base of customers.
A huge risk for Ford to take, but to re-hash Explorer any other way would have regulated it to "also ran" status.
As an aside: Chevrolet Volt deserves the NA Car of the Year award due to it's likewise re-writing of what is to be expected of EV's....right, wrong or otherwise...it is also a game changer in the market and eveyrone is watching it