Online communities are not permanent residences... not for most people. It is normal for people to come and go. It is also normal for people who have been long time members to long for days gone by.
"Get offa' my lawn!"
Here it is pretty simple, as I see it.
The people with the most questions are pre-transitioners.
The people with the most to say are giddy, new transitioners.
People who have been transitioning for a while or are post transition are the wisest about certain topics, but have also, already answered that question 49 times.
This is a a site dedicated to trans issues, and the prefix trans has a specific meaning... it suggests a process of going from one situation to another.
There are not a lot of trans role models, in part because a sussessful transition often culminates in resuming life... many of us just want to blend in to society rather than stand out, and once that is accomplished, thoughts of our pre-transition self have begun to fade in various ways. Our need to communicate with the trans community can diminish.
In short, the people with the most to offer, have the least need to offer it, so if they feel like people have ganged up on them, they get smited one too many times, or their friends have vanished, it makes it easier to leave the community.