Dan Brown was a semi-popular summer author before Da Vinci Code. And Da Vinci code exploded well before the Catholic Church got all pissy, thus increasing his exposure. Anyway, it's inaccurate to claim that he would not be popular without the controversy. Da Vinci Code is basically the Name of the Rose (Eco) for the wider lay audience. It's an easy detective sort of read, without the literary implications and stylistic twists that tend to confound the casual reader with an author like Eco.
Brown and Rowling have reached a mass audience--many ages, and far more than a simple stratified fan base. For whatever reason, they seem to appeal to all personalities. Tolkien has, and will always only remain popular to a niche crowd. The movies have exposed him to a wider audience, sure, but he really only appeals to the fantasy/D&D, younger crowd (and those that have grown, and fondly remember reading him when young). He simply doesn't have as wide appeal and never will. Assumptions otherwise just speak of an unwillingness to see beyond the fan appeal. There's nothing wrong with that, I'm just saying it's hard to see beyond his limited appeal when one invests so much attachment into something that they love so much. Naturally, a fan would assume anyone else would love it just as much.
Again, I much prefer Tolkien, but in terms of literary merit, his style isn't any more complex than what you get with someone like Dan Brown and unfortunately, his characters are just as flat, and perhaps even more typed, which sucks. He did exist within academia, but was always sort of a dark horse, trying harder than he should have to really get "in." But he always stuck to his guns despite the advice of his colleagues to stay away from "fantasy trivials." Few people consider the fact that de didn't even have a fan base at that point, so he was literally on his own, with no one to please but the academics whom he was determined to prove wrong--but would essentially fail at doing. That takes balls.
The only thing I'm trying to say is that calling Tolkien a literary stalwart is like calling Jim Morrison a poet. Fans will always say such things because their frame of reference--their ability to actually compare among many artists (or whatever) of the claimed genre--is quite minimal. Tolkien is a god in the fantasy/sci-fi, young adult realm that simply would not exist as it does today without him. He is barely a blip in the wide literary map, however.