I believe there are a few reasons. First, I suspect that with four engines the 747 has more power-generation capability, and probably has better flight characteristics and range after the loss of one engine than would be the case with the 777.
Also, by sticking with the same airframe they’re not having to “reinvent the wheel” to accommodate the technology already incorporated into the current 747 frame.
Fuel efficiency is probably not the biggest consideration because of the air-refueling capability of the aircraft.
Just my opinion...
Air refueling has been deleted from the replacement because it was never needed with the current 747s.