Maybe because he created truly believable fantasyworld with it's own history and geography and languages? Not to mention it's an entertaining ride to take. Those books I mean.
Well ok, I ain't that keen on fantasy literature, so maybe I'm not the right person to dig deep into the reason why that trilogy's so popular, but I did read them and enjoyed the hell out of 'em.
Now, I should pick up that American Psycho for the fourth time