^Well, except that the Full Circle fleet wasn't all together at any one time. Usually it was split up into thirds, or would've been if it had existed long enough to have a "usually." So they could've just acknowledged that the other ships were out there but focused just on Voyager and its two support ships. So whatever the reason was for this change, I don't think that was it.
Well I guess reading over my comments I could be implying that Kirsten was being lazy... and 86'd the fleet to save trouble. Seeing as she tackled the Janeway issue, AND wrote 4 books that averaged over 400 pages I would never call her lazy.
However when a plot contains a giant, implacable foe with the capability to destroy everything and everyone, it helps to have a few extra ships for the foe to destroy/absorb and spare the main characters. The fact that Ms. Beyer wrote in so many vessels and their commanders, lieutenants, engineers, etc. allowed for this kind of story to be told and still have some emotional resonance. Which is cool.