I remember back in the day (late-90s) the B. Dalton in our mall carried almost every extant number TNG, DS9 and VOY novel and about half of the numbered TOS, so there was nearly a full unit of Trek next to a nearly full unit of Star Wars.
Now, Star Wars is an evergreen franchise. I don't think any of those books have never not been in print (Well, after Thrawn. I know Splinter of the Mind's Eye was OOP for a while.) till this day you can find books first printed in the 90s with updated timelines, baclkists and adds in them. So, when I go to my Barnes and Noble - which replaced my small B. Dalton with a huge, two-floor store just outside the mall - I'm not surprised to see 4-5 shelves of Star Wars novels, containing books that range from just out this week (Scourge!) to something nearly 20 years old, with the original cover art.
Star Trek, on the other hand, commands barely 2 shelves. In general the books on the shelf are no more than 3 years old, but more often then not are really only from within the last year. I have noticed that recently a bunch of older Titan books have popped up which were never around before. Part of this does have to do with what's in print vs. out of print, but it does seem like Trek is a "You're in or you're out" type deal. If you want to do any backlist reading, you have to look elsewhere. I don't know that's a sign of disinterest per se, but more to do with the audiences (~8 years since the last series aired, 1 movie ~3 years ago vs. an ongoing cartoon.) I do know my B&N does not stock the Abrams-verse Starfleet Academy books at all, which is curious.
Now, granted, I do know that Wars puts out 4-5 new books a year and Trek puts out 10-12, so there are turn-over concerns and the like.
I don't know that the interest is drying up, per se, but the audiences are vastly different in size, interest, etc
A second hand bookshop by me that had tons of 80s/90s Trek material recently closed, so I really only have the Kindle as a way to get into anything that's not 1.5-3 years old.