Part of it is the nature of the hobby. There are only a few kinds of questions that can be asked and most of the answers just throw the issue back into the asker's lap.
Q: Something's wrong, how do I fix it?
A: If it's a common problem, look it up. If not, figure it out yourself because no one knows.
Q: Can I put A with B?
A: Yes, and a quick search would have told you that. / No, and a quick search would have told you that. / No one's tried it, so you're on your own.
Q: My ______ is sick, but I don't have enough experience to diagnose it.
A: It's Common Disease XZY, treat it with Medicine ABC. / It's Incurable Disease ZXY, sorry about your first fish. / It's New Tank Syndrome, do more water changes than you have time for. / That's weird. First tell us everything you've ever done to the tank, then do a bunch of tests, then hope it doesn't die before Special Order Medicine ABC can arrive.
Q: I want to try [ambitious project]. Has anyone else done it?
A: No, because it's really hard and likely to fail. But please post pics - you'll get lots of admiring comments, but no real advice.
Q: Look at my awesome tank!
A: Wow, that tank is too awesome for me to comment more than three words! / I can see a dozen flaws but you didn't say you were open to critique. Uh, nice guppies.
Q: Which is better, A or B?
A: A is better for me, but B is good, too. You'll have to decide for yourself.
And so on.
My favorite threads are controversial topics where there's really intense discussion and you have to really think about stuff you've never considered before. But those threads are tricky to keep from blowing up. And there aren't a whole lot of topics like that in this hobby. It's very technical - either something works and there's tons of evidence for it, or it doesn't and there's a lot of evidence against it.
The next best thing is when someone makes a detailed journal of an experiment they're running and the results are completely different from what we thought would happen. But most people don't have the resources to do those experiments or they don't finish them. I loved the thread where a member just kept adding more and more ammonia to an empty tank to see how much the bacteria could process. I've been wanting to do that experiment since I got into the hobby, but I never had the time.
Another part of it is that the forum itself is pretty rigid so it hasn't kept up with the times. Look at how many site announcements/updates get made in a year - hardly any. I know they're doing a lot of behind-the-scenes work, but the stuff we actually see never seems to change. Can you fault people for leaving when the articles are outdated, the plant profiles are not very useful (WPG instead of PAR, background/midground/foreground instead of actual height, can't search by attributes, etc.), there aren't enough stickies, and the subforums are both too narrow and too broad? (There are a lot of questions that fall under more than one subforum, and some that don't seem to fit into any.)
In order for the forum to stay useful, it needs frequent tweaking. Ideally, you crowdsource it, so that the people who are bothered by a problem are the ones who help solve it (e.g. have a list of articles that need to be rewritten). But (and I may be wrong on this; I'm not as active as I used to be and things might have changed a bit) there's an attitude of "we have enough to do keeping trolls under control, if _____ bothers you that much then you can go to another forum". When the mods refuse to change anything minor because it's minor, people stop offering suggestions and the forum loses the cumulative effect of many minor improvements. Then, when they finally decide to address something, it takes a lot more effort. Instead of minor improvements made every couple weeks or months, you get a shocking overhaul every couple years or no change at all.
For example: a while back I noticed there were a couple of art threads on the forum, so in
this thread I asked them to change the wording (just the wording - no code!) of the description of the photography forum to allow/encourage artists to post their art. It was shut down immediately without any room for discussion (and in the rules it says you can get suspended for arguing with mods in the forum, but they wouldn't answer my PM when I asked for an explanation). It left a very bad taste in my mouth.
I have other suggestions for improving the site, but after that experience, why would I bother?
Or like
here. A member wanted to help the site by creating/expanding on a profile. When it didn't work, he asked for help and the mod said, "Unfortunately [...] some of the existing functionality has stopped to work as intended, and has not been restored so far.
If you have an interest in getting the fish and plant profiles restored, accessible, and update-able, post a thread in the "Planted Help Desk" section for the admins to respond to. Thanks!"
In other words, when the member wanted to do something to help the site and the site didn't work, instead of the mods taking notice and fixing it, they told him to make a request to maybe get it fixed. ???
Now, I admit, I know nothing about running a forum except that it is not easy. AFAIK, none of the mods are paid and they're busy fixing lot of things we never see. But it's hard not to get frustrated when the things we
can see are neglected.
In a nutshell, the reason there aren't as many active users is that the hobby itself is kind of a one-man show, and this forum in particular has lost some of its usefulness.