I could be wrong, but part of the problem is probably that there's only really one current Triple-A RTS title with any mindshare these days, and that's Starcraft II. The Age of Empires series has been gone in its real form for years, along with Empire Earth and Rise of Nations. Thus the genre is poorly represented, providing few *major* options that a lot of people know about, and that one primary option feels very different from the more casual style of AoE.
It should also be noted that Ensemble Studios (and thus the AoE series) was eliminated not because of commercial failure, but because it evidently fit Microsoft's strategy (though I don't really get that), and the IP has not seen a major new entry since (Age of Empires Online was very unsuccessful. This is what they get for trying to turn AoE into a weird F2P social game thing)