Why hasn't Boeing updated the 747 with technology from the 787?
They did, in the 747-8:

Wikipedia:


  Similar in nature to the 747X, the stretched 747 Advanced used technology from the Boeing 787 Dreamliner to modernize the design and its systems. On November 14, 2005, Boeing announced that it was launching the 747 Advanced as the "Boeing 747-8"


(Emphasis mine)

The 747-8 includes technology in the cockpit, cabin, wings, and engines borrowed or learned from the 787 and 777 aircraft programs. Obviously you can't just pull the wings off of a 787 and stick them on a 747, but the wings were redesigned with aerodynamic information learned in the 777/787 program.