I think the biggest issue that kind of conspired against the 747X series was the fact the plane did not offer any speed advantages over the current 747-400, nor did it offer a massive increase in seating space like the A380 offered.
However, it appears Boeing may be preparing to give new life to the 747. There is now talk of a 747 using the fuselage length of the 747X (not 747X Stretch), with slight area ruling of the fuselage, new wing with either raked wingtips or Aviation Partners winglets, and the choice of GP7200 or Trent 900 series engines. This will result in a 747 that carries 440 passengers cruising at circa Mach 0.90 with a range of nearly 9,000 nautical miles. A freighter variant off this new 747 could carry a full payload over 6,500 nautical miles, eliminating the Anchorage, AK (ANC) stopover for fuel on transpacific flights.
I think because of the very long range and very high cruise speed of the proposed 747 variant, once the economy picks up again expect a fairly large number of airlines to be quite interested in buying this new 747. This would essentially give most of the advantages of the Sonic Cruiser proposal, but would actually carry more passengers than the current 744 and can just about fly the LHR-SYD route non-stop. Another important advantage is the fact this proposed new 747 would require half the development cost of the A380, so Boeing could price the plane quite competitively.