I would suspect that many of the older members are just bored of TFF and moved on to other forums or other hobbies. An aweful lot will have moved into more specialised area's and joined more related forums, be it for predators, marine or planted tanks which all have forums far better suited for people specialising than TFF.
As has been said it can get tiresome answering the same questions over and over again and they tend to go down one of two routes, either telling people to use the search button or more often simply not replying.
Many (who aren't changing) will also have stable tanks with few if any problems, so have no questions to ask themselves and if they have, they'll probably search for the answer first and very often find it without posting.
So you have lots of members who are bored of answering the same questions again and have no questions of their own to ask, if you aren't posting, why stay around?
Others get "promoted" to mods, some of whom stick around, others seem to dissapear not long after. Too many good, friendly, helpful and knowledgable members have become mods only to dissapear a few months later.
When I'm meeting other financial advisers, once we've finished talking business, some will simply leave the meeting, others will stick around and discuss other stuff, "general chit chat" if you like
getting to know each other better and in the process developing friendships. If someone then thinks of a work related question later on, those who have stuck around to discuss football, politics, cars etc can join in another work related discussion, those who left have gone and we lose their expertise.
I am a member on other fish forums, diving forums and professional forums, this is the only one which doesn't have a general chat area and the only one which the moderators are not capable of running one. The decision not to have one is fine, but I believe one of the consequences of that decision is that experienced aquarists have nowhere to hang out, whilst staying on the forum, meaning they are here to help when something new crops up. As there is nothing to keep them on TFF, they log off and will log on less frequently until eventually they just stop logging on.
Arfie