My inner cynic looks at the popularity of Hot Topic and its ilk and says 'there you go', as far as the RQ's popularity goes. She's a goddess of ~Death~, oooh. (My inner cynic and the inner cynic of most people I know actually, but)
It's bizzare, because nobody ever seemed particularly interested in say, Wee Jas or Sehanine Moonbow, both of whom had associations with death, but suddenly RQ is the best thing evar. *shrug*
Personally I don't find her appealing at all so that may colour my perceptions a bit.

I didn't find Wee Jas or Sehanine Moonbow particularly interesting either, btw. Though my views on death in the context of D&D are more or less 'Yeah, so? It happens. Why are you making such a big deal about it?', so death gods tend not to appeal to me.
Though I suppose it might have something to do with the undefined afterlife of 4e. Earlier editions, unless your soul was trapped or destroyed somehow, your soul just plain went to your patron god's realm when you died. Now you go to the Shadowfell and maybe your god liked you enough to actually give a damn about your soul, maybe you stop existing, or you pass onto some nebulous other thing, or you linger in the shadowfell or whatever claptrap. (I don't like 4e's afterlife, no. I have ambiguity in real life, I *like* it if my fantasy stuff has some definitive answer if only for the sake of rules stuff. Nothing says the average person or even your character has to know of this, if you want ambiguity...)
I want the new Corellon next, barring that, one of the entirely new gods other than RQ. We know literally nothing about most of them other than the single paragraph, and there aren't really previous versions of them to draw inspiration from.