Keith, for me it was that I ended up making an unscheduled trip to the St. Louis area for my father's open heart surgery. My brain was in no shape for discussion.
Also, as non-Baptist now, often the topics just aren't about subjects/situations I'm involved in any more.
I've also noticed we don't get many new participants. Part of that I'd chalk up to the fact that PHP BBS systems aren't that popular any more. Most discussion groups I am in that used to be groups like this are now on Facebook. Personally I think this is a more useful system. But Facebook is now dominating online discussion groups.