Others know better how to answer your question fully. Of course all of the reasons you list are true to one degree or another.
For some reason, your post really made me miss Marco. Nothing against the current regime - I just really liked him both as a person and his contribution to TrekLit. Seasons change, people move on. Sigh.
I miss S.D. Perry. What is she up to these days? Her DS9 Relaunch novels were the ones I loved far and above all the others.
I miss KRAD too. I know he's got plenty on his plate these days. I think of him as the Ron Moore of the lit world, though - that idea-guy not afraid to pick some fights and push for great stories.
Sure wish we could get Andy Robinson back for more Garak. And I'd love to see the Reeves-Stevens do more non-Kirk-related work. The Millennium Trilogy was awesome.
What about the old stand-bys? When was the last time we saw a Friedman installment? Death in Winter? That was nearly 5 years ago. Considering how he used to crank 'em out, it's like he dropped off the planet all of a sudden.