Well, I grew up on TOS so it's still near and dear to my heart. Don't get me wrong; I watched all the later shows religiously while they were on the air, but TOS is the only Trek series I own in its entirety or watch with any regularity.
Plus, part of the appeal of writing 5YM books is that I can basically go off and do my own thing without having to worry about meshing my plot with some complicated, over-arching continuity. I can see where that kind of collaborative enterprise can be stimulating in its own way (I enjoyed working out 4400 story arcs with Dave Mack), but there's something to be said for being able to work independently and explore my own favorite corners of the Trek universe without having to coordinate my efforts with eight other authors! Just as a personal preference.
Besides, its seems to me that the 24th century is in good hands these days!