Mystery has historically been the mainstream of publishers. Now romance is, of course, pushing it aside. But mystery requires cerebral exercise. At least good mystery does. I don't know about romance.
Romance is much more of an emotional thing.
The mystery demands some participation by the reader. Another nice thing that a well-written mystery novel tries to do is educate the reader simultaneously about some aspects of the world. So I take them on travelogues or tell them how to find somebody who's impossible to find, throwing in at the same time tons of arcane but interesting minutiae. But I was able to get my hands on the blueprints for that library, so every door is in the right place. However, the rare manuscripts room is not in the same place, so, anybody who wants to get their hands on a rare manuscript, don't use my novel for a roadmap.