My first reason is lack of time. I teach, and am no longer as strong as I was some years back, so I keep my energy and my time for researching and writing Hubs, and commenting which I enjoy a lot.
I have read through some threads in the Forum, and it seems to me that several start of fine, and then seem to fade in interest. And I've also noticed that some threads get pretty rough, with harsh comments. I have no time for that.
I like the comments and the questions much more.