Why does Brown taking the utter crack that is the Gnostic Gospels and treating it as actual secret history and theology bother me? Why does it bother me that he has a conspiracy theorist's romp with major intellectuals and secret societies of the Middle Ages and Renaissance--the same romp that several other authors who wrote stories about the Illuminati or the Cthulhu Mythos or whatever did? It didn't bother me when H. P. Lovecraft played fast and loose with certain Renaissance societies, books and characters--why does it bother me that Dan Brown did?

Because when those other authors did so, they were not really presenting their stories as fact. It was clearly understood that their pieces were true fiction, were written as fiction, and were presented as fiction, with their backgrounds understood to be fact in that fantasy setting and not in the real world.

Dan Brown did no such thing, and did, initially, attempt to pass his fiction off as "romanticized fact". He has been seen in interviews attempting to claim that he researched real facts which he presented in his book to bolster the story, when, in fact, he made these things up whole cloth, and simply tossed in njust enough reality to make his fantasy plausible.

In short, he was trying to pull a fast one on the public in an effort to boost sales. The other authors were simply good, and didn't need to trick the public into thinking that their work was "real" ro sell their product. They were a bit more honest about their lying.