There was a brief period where ignorance reigned and Dan Brown was considered an actual historical writer. That drove a lot of book sales, when in fact Dan Brown doesn't write anything remotely historical.

I think the issue is that, for a moment, people legit thought of him as a good writer/his books having actual research etc. Then he got too popular and people started ripping his work apart because they got tired of hearing how good his books were etc. It's a pretty normal cycle for a pop author.

As for comparing him to Hitler, people gonna be people.

An alternative is that he isn’t on that list because he’s a bad person, like Hitler. He’s on a list of people GP doesn’t want to read. He doesn’t want to read Hitler because Hitler is a bad person, he doesn’t want to read Dan Brown because Dan Brown’s prose is clumsy and not worth reading, and he doesn’t want to read Ayn Rand for both reasons. (Sorry for the dig at the end. I think that’s funny).