I'm reminded of something I read recently about comic books. It used to be that you could find comics at the corner drugstore or newsstand or grocery store, and readership of comics was in the millions. But then the direct-market distribution system took over, and comics ended up being sold primarily in specialty comics shops and disappeared from those other places, and today comics readership is in the tens of thousands. It wasn't because the audience lost interest, it was because the distribution system changed in a way that targeted the product only at active, invested fans and removed everyone else's access to it.
I wonder if a similar mechanism is in place here. Maybe it's not that readers have lost interest in Trek books, but simply that they've lost casual access to them due to changes in the distribution system, i.e. the dominance of online book buying.