I think as was mentioned, some of those who were DF are still in pain regardless of how long ago it was. Posting about those feelings means reliving them. One of my close family members is that way.
I know many who have left that were never baptized. They were raised in the "truth" or around it and that in itself has seriously damaged them. They don't want to talk or think about it. This must apply even more so those who were officially disfellowshipped.
Another reason, as was also brought up are those who have been DF, yet are going back. I personally know some of them don't have any disagreement with the WT organization. Unfortunately, they feel they have sinned and accept the judicial committees decision. They sit meekly in the back row until they are reinstated. These wouldn't be participating or even looking at ex-jw sites online, as they don't consider themselves as such. Maybe someday, they will see the TTATT and find support groups such as this. At that point it could be a lifesaver for them as it has been for me. I'm one who really needs to express my feelings, have them validated by others (like on this forum) who have gone through this and come out to live happier lives.