One of the main reasons is that 3d talent for this sort of thing is pretty clickish. Occasionally you might have crossover or get permission from another group to supplement your ship roster, but ideally you want people on board who understand the game the meshes are to work in.
In regards to FS2, everyone and their mother (not literally, just poking fun) has their "Dream Campaign" or a TC idea. Unfortunately 99% of them never even get as far as we have in our respective goals.
The modeling talent either is busy with their own things or latched onto other projects. Most of the time they keep on plugging and hang in to work on them but you get stragglers and those who end up just falling out.
This happens on individual basis or even at time whole projects.
Hang tuff, stay true to your goals, and always look for opportunity. We work on NO ONE's timetable! Those who can DO , those who can't BITCH!
I try not to bitch, whine a little sure, but we all do that on occasion!