Another thing to consider is that while there are popular books with non-white MCs, not all of them include POC models (or illustrations of POC people) on the covers.
For example, until shamefully recently, I was under the impression that Patterson's character Alex Cross was a white woman, and the covers of the Alex Cross novels I've seen haven't done anything to dispute that. I obviously have not read the books. Even if I'd read the blurbs, I probably would have picked up on the fact that Cross is a guy sooner. But just based on the covers I casually saw, I was able to keep up my erroneous impression for a while.
I think there may very well be readers who would be fine with reading a book about a POC character, but might subconsciously avoid books that have them on the cover. It seems like a lot of books with POC models on the cover are marketed as urban fiction, or as books that deal heavily with race, and I think that can lead readers to form unconscious impressions about what a book will be about.
So I think it can happen. But this book isn't the best example to look at considering the technical issues and the fact that there's a creepy-looking white guy on the cover, as well. It's always possible that race could be a factor, but there are so many other variables.