What I've found out about Fanfiction is it tends to stem from the audience's plan to fix a facet of the show/book/comic/media that annoyed them. Series that get lots of FF tends towards those that while being in an interesting world or setting have plot points people felt was misused and need to be repaired. Some times yaoi fangirls get involved and they decide the problem with the series was the lack of every single male on the show not being official romantic couples. This is why of the 15K+ fanfics for Gundam Wing on FF.net... well over half of them exist only to pair off the male cast members with each other. Writing quality is often at the same level as the premise. In general fanfiction written only to ship a particular couple tends to fail in quality and content as the only point of it was pairing those two off and the thought going into the work ends right there.
This is also why so many really good series end up with no fanfics. There wasn't anything anyone wanted to 'fix' so little to no work is done by he fanbase.
Worm is an apparently engaging series... I've not read it as I've heard the first part is a miserable slog of torment to get through. Its got all the elements that attract people to the story and generate Fanfiction. Developed characters. An interesting setting. A super power system that can be diverged a million billion ways without hurting the main essence of the story. Grim Dark that you can work around without requiring that some one on the literal other side of the world does something stupid and feeds everyone to horrors from the beyond accidentally, if the happiness of the story gets to much of a foothold. As a law of physics.
FoZ itself was largely considered not worth dealing with for years. The main issue being that its wasn't the setting that was the issue so much as Louise being herself that caused the issues. Louise (fairly or not) was considered as a 2000s era female lead for a chick flick. See in the '90s a chick flick was characterized by having one person die slowly and using this as vehicle for cathartic feels. In the 2000s the model changed. The basic plot became: Female lead with as few redeeming characteristics and personality traits as thematically possible is completely undatable. So she dates anyway and whatever guy she is inflicting herself on is battered and twisted until he becomes 'perfect' for her. The less effort she puts into the relationship and the more undatable she is the more stable the relationship ends up.
Whether Louise actually is one of these undatable girls that gets human male clay and warps him to her 'needs' while remaining unchanged at the beginning of the story is a judgement call. Though that is the literal true of a later clone of her named Palm Top Tiger.
No one really touched the series until 'Hill of Swords' got written and people found the fulcrum of change in Louise summoning a different familiar. Now people admit FoZ exists to have Fanfics of it.
