What I've come to realize is that for the most part sci-fi and fantasy writers depend on certain tropes and styles of writing and rarely step outside of those tropes and styles to examine other types of fiction. Patrick Rothfuss writes very well, but what sets "Name of the wind" from most other fantasy and has distinguished him so much among the fans of fantasy fiction is not the originality of his world (because it's hardly original), but how he writes. He exposes the typical sci-fi/fantasy fiction reader to styles and tropes that are not typical to these genres, therefore setting himself apart and appearing original. Same reason I suspect that Mieville has had to much success.
While Rothfuss looks to autobiographical/storytelling styles of fiction, Mieville looks to Pahlaniuk's urban fiction/stream of consciousness/minutae OCD style. Neither are found in sci-fi/fantasy very often, and thus both appear as a breath of fresh air.the styles and tropes in mainstream fiction have changed a lot since then. And personally I find that far too many sci-fi authors are still writing like Asimov and Clarke and Heinlein and Vance were still alive (which would have been bold and original - and was - back in the day), while too many fantasy authors write quite blandly. Rothfuss, Mieville, Stross (I'm talking about his more out-there books rather than Laundry Files books), Vance, Gaiman, Susanna Clarke, Ursula LeGuin, and several others look outside of the established sci-fi/fantasy/horror mielieu for different styles and tropes. That's what differentiates them from yet more "elves with swords" or yet another "military space opera with blasters and FTL travel and aliens" (here's looking at you Tanya Huff) that cram bookstore shelves.