1. Too small & not enough engines. USAF demands 4 engines.
2. 95%+ of people would not know the difference between a 787 and any other widebody twin-jet.
And separately, the 747-8 has new wings, engines, avionics, flight controls, and lots more. The only thing not new and "cutting edge" is the familiar (and arguably iconic) humped shape of the fuselage. While most airliners all look alike, the 747 is immediately recognizable (OK, the DC-10/MD-11's shape is also kind of unique and easy to recognize.)