It's really a balance between here and there. Here, the rules are far less strict, but there have been arguments in the past that have made me, personally uncomfortable posting, worried about blow back (I experienced my first internet flaming here, from a now banned member). While GC has some pretty strict rules which are enforced, but some people really like having that structure and safety.
To be fair, one big reason another fan forum is inactive is an inescapable, undeniable truth that active Sailor Moon fans don't want to face.
Sailor Moon is not popular...and is decreasing in popularity.
This might seem to be at odds with me saying it's profitable, but it is only profitable today because the same five fans obsessively keep buying the tie-in products. The movies didn't do well in the box office and I suspect the media sales are disappointing. Why? Because it's the same old material regurgitated, except not even the same as it was for nostalgia sake.
Besides, most people use Reddit these days if they want to have in-depth discussion, and the Sailor Moon subreddit is fairly inactive. While 24K subscribers sounds like a lot, only 100 are those are active, the subreddit focuses mostly on fanart, videos, photos, collections, and the occasional post.
Tumblr used to be big years ago, but between general exodus and the fact that its very nature prevents discussion, you're not going to get much discussion there. If someone doesn't like you they block you, then they wonder why no one gives them feedback.
It also doesn't help that the Sailor Moon community has a bad reputation of not tolerating dissent. Has it gotten better? Absolutely and by leaps and bounds. But when there are stories of being prevented from "Sailor Scout" or registering in the name of Sailor Moon character, then the mere legacy of these restrictions prevent people from entering discussions at all. Unlike other fandoms with active products, those arguments are a lasting barrier to discussion.
Still, the main problem is popularity. The irony is that when fan projects do seem to take off with outsized success via shared factors, one path to revitalizing this franchise seems crystal clear, but it is the one path that existing fans have rejected. So this is the franchise you wanted; a franchise in a dead fandom.
I mean, they revived She-Ra and Voltron, two lesser and older franchises to much success. Sailor Moon has way more potential. But it will never achieve it at this rate. And we will have no one to talk to but ourselves.