If they once owned thousands of Bitcoins, and sold it all for a couple hundred bucks, I can imagine they don't ever want to be reminded of "what could have been", and don't want to visit Bitcointalk anymore.
I'm not even in that category, and yet sometimes I still lament what I could have had if I'd just bought and held bitcoin when it was available for $2-400. I seriously can't think about it for long without going into full despair mode.
I used to know this fact but forgot it: when did signature campaigns really kick off here? For some reason I thought it was before 2012 (I recall reading the first thread in which signature space was offered up for advertising), but if that's true then the membership count given in the OP seems really low. I'd be really interested in seeing a graph of time vs. member count with the start of sig campaigns marked on it. Probably membership exploded, but I'm not certain about that.
Anyway nice data presentation, OP. I think I'm out of sMerits, but I'll check and if so will give you some.