With respect, and I mean this constructively, signing out for that reason seems a bit counter-productive. Think of it this way: you were put off because the grid looked small - when you sign out it looks even smaller and others are even less inclined to sign up.
I think the Formula Reiza's are super fun; when I did testing, I think I must have done about three hours on this track with these cars just because I was having a blast. There are a few who sign up, but in general, and unfortunately, they are poorly supported events. If a race at Spa doesn't inspire people to sign up to them, I fear nothing will.