JohnGrey, Graham, Claudel, Raoul76, Caminus, PilgrimageofGrace, Ethelred, PereJoseph, Telesphorus ... those are the types I find are missing. They probably have high IQs. BTNYC is the only person posting recently of that caliber.
Most of the members listed are not banned. Raoul flipped in some way -- I don't remember the details (so don't quote me) but I don't know if he's Trad anymore. Except for Tele, who I banned and then un-banned later (but he understandably didn't come back), all the others left on their own.
Several of these left for the first edition of the "Archbishop Lefebvre Forum" started by ServusSpiritusSancti (who is another enigma in the Trad world, BTW. No one has heard from him in years) After that forum shut down, a couple others tried to carry the torch, but none of them was ever 1/20th as popular or active as the original. Both successors today are basically or literally dead. But even when these forums dead-ended, the members never came back to CathInfo. Go figure.
Why do so many good members leave CathInfo? I have no idea. The reason is different for each member. Perhaps they need more time to earn a living for their families. Raising a Trad Catholic family, or even just living a Trad Catholic life, in the 21st century is not an easy task.
You say "they probably have high IQs" -- but you're not sure? First of all, many members on CI have "high" or above-average IQs. But I can't force high IQ people to stay, or to spend X hours a week on CI.
I don't know what you're implying here -- because I certainly can't think of any fundamental
reason why high IQ people would tend to leave CI in significant numbers.
But as for personal/individual reasons for leaving CathInfo, there are a million reasons, let's face it. Reversion to the Novus Ordo and/or apostasy, marital trouble (up to and including divorce), nervous/mental/psychological breakdown, personal conflict(s) with other members, crisis of Faith, corruption by the world, personal weakness, lack of discipline, burnout, new job, new living conditions, new family situation, health crisis, financial difficulties, new hobby, new fervor in some other area of life/hobby, just to name a few!