I had a friend live in Bulgaria, learned to speak Bulgarian and everything. Her elementary school students were very sad for her that she was like 24 and not married. They thought no one would want her by then.
Ha, my kids felt the same way about me.
So, I feel like we have this discussion almost every six months, but at least now we have some quantitate data surrounding it. Beyond Civ itself becoming less popular, I don't think the reasons for this are rocket science.
1) The product itself isn't very good. By this, I don't mean the community, I mean the blood and guts of CFC. This layout and format essentially hasn't changed at all since I started coming here about a decade ago. Few new features have been added, even when we've asked for them (like a like button) and if anything, thanks to forum splitting, the actual user experience has gotten worse. As other communities and other technologies have grown, there is less a reason to come here. There are probably upgrades that would make this a more enjoyable posting experience, but they either cost money or time, and I doubt that the powers that be are looking to make any investments. Which is fine.
Lots of people have mentioned that they go to Reddit now instead. For me, I go to Twitter. That didn't really exist during my heyday here, and I found a bunch of news stories I would have never otherwise found here. Now, I have a zillion other places to go.
2) The core group has aged out. The college-aged and young professionals of the 2008ish era are now less young professionals. I joined this place when I was a senior in high school, and now I've graduated college, had several jobs, got married, and have a kid. Many of my peers during that era were around the same age, and have made similar life changes. That means less posting, and if you don't have a ton of inertia built up in this place, there aren't attractive reasons for making the time investment.
3) It's boring. As others have said, the moderation can be tough, frustrating and arbitrary. Most of these threads were political, and after several years, we've already beaten those horses to death. The demographics of the forum make discussing some topics really frustrating (we had a run where we had a ton of racial-issue threads...now, feminism seems to be popular. Both were horrid), and most things feel like re-treads.
The only solution is to start topics that are actually interesting, or new.