I think many of them just aren't intrested in learning a new flight model/game. They may have tried a 2 week trial, and got thier tails handed to them, and decided they didn't want to have to re-learn a game and the flight models.
S3 is a very well organized and structured event, I enjoyed it for many years. But the lack of development and support eventually drove me to looking for a different alternative. Of course, the organization was based around 60-100 players per side, and much easier to deal with than say 250 players per side.
The S3 had (when I flew) more involvement and details associated with it compared to FSO. There was an additional cost to flying the S3, unless you had the unlimited account which was $20-25 bucks a month if I remember correctly.
The single biggest difference between FSO and S3's that I can differentiate, is that there is a definate focus on remaining alive and keeping your plane in working order. Penalties and shortages of aircraft, especially better ones, was a definate issue in many S3. If your side foolishly wasted your aircraft, then the better model planes weren't available in the later frames. When there's 5 frames, 3 hrs long each, that turns into a real diliema sometimes.