3 gun matches should be FUN!
Matches should have stages designed for the average shooter to be able to accomplish.
Many years ago I listened to some seasoned shooters talking about the match that had, in their opinion, too many shooters that had timed out. I believe that a match that has more than 5% of the shooters time out on a stage, the stage is either too long or too hard. Remember the average shooter. Most of the shooters in these matches are not the gee-whiz, smokin' hot aces.
Some match directors have stages that are SG heavy which leads to a longer re-set time which in turn makes the match schedule harder to control. As a personal note, I do not like a SG heavy stage. That's just me personally. (Of course I also shoot quail on the ground.)
As to the rest of the reasons folks are not participating, I think it might be to the popularity of the Precision Rifle Series of matches that have blossomed throughout the country.