[Simlasa]

I think COC's strength is a fairly cohesive and colorful mythos... and the definitive flavor of the 20's and the pulp/jazz age (though it seems like modern settings are just as popular). I think it's been a good seller in SPITE of the spectre of near-certain character demise/insanity (though I don't think it has to be as common as a lot of folks seem to assume).
Maybe people mostly play it as one-offs... like Paranoia? Maybe they tweak it so the PCs can rape/loot/pillage the Old Ones into being just like orcs in D&D?

Dark Conspiracy looked great to me at first... but it never felt like it had that same cohesive feel. All these disparate urban-legends thrown together... seemingly without rhyme or reason (it's been a while so maybe I'm forgetting the reason... but that would suggest even if there was one it was forgettable).
I liked the attempts at 40's styling on some things... but that seemed like window dressing that didn't permeate the game and was easily overlooked.

A better example of that sort that got lots of good press was Unknown Armies. I'm not sure how popular it is but after reading it I felt like it was an attempt to make COC more cinematic and add some meta-gaming dice gimmicks. In ways UA is darker than COC while still letting the PCs be more 'heroic'...
Still, I don't see it arising to the levels of popularity COC has enjoyed... but then that would support your argument.