The only thing I can think of is that everyone loves a great conspiracy. And this book probably also appeals to skeptics and people who don't like the Catholic Church.
James L Garlow, PhD, and Peter Jones, PhD, both scholars of impecable credentials have written a book called Cracking DaVinci's Code which is really, as the author's note, cracking Brown's code. And what a "Code" it is.
They clearly point out from tha author's own words, that he didn't intend it as "just a novel." Instead it is a propganic tract, meant to sway, misinform and alter the thinking in as many as it can.
Brown on the Today Show insisted that other than his fictional character that is used to pilot the novel, the rest is absolutely fact, in his words. The book is great in its analysis of the travesty and of the behind the curtain leftist pagan crap it is trying to promote. Check out the following related sites:
www.jim garlow.com
www.cwipp.org
www.breakingthedavincicode.com
As I haven't checked them out yet, I haven't provided a live link...cut and paste if you chose to explore.