Outside that it really only takes one dedicated enough person to set up basic tools, I've seen it happen in plenty of communities. People just generally don't care enough for smut games, TiTs barely has mods either, even CoC only had a couple of real projects from what I know (hgg, xianxia and expanded?).
Is definitely a more complicated thing than just "nobody made any effort" though, I agree.
I think the difference in environment is also important.
When a dev releases a few decently-documented tools of their own volition, and says "this game supports mods, go forth with my blessing as long as it's not-for-profit", then there's barely any barrier to entry.
If mods still don't spring up at this point, then it could be that:
1) The game or engine are still mod-unfriendly despite the dev's best efforts
2) The playerbase is small enough that the subset of players willing to create mods is close to zero (and this group is much smaller than the one which would
use mods if they existed)
3) The dev is very open to suggestions and many things you'd want to mod in get added eventually, making mods a low priority
(All three things can apply at once, to varying degrees.)
But when the devs actively (and rudely) shut down any mention of modding, and withhold modding resources like Gollum clinging to the One Ring, it makes it hard to muster any interest. Why jump through additional hoops to (subjectively) improve a game, when the devs will spit on you while their sycophants cheer them on?
It would make more sense to focus modding efforts on a game where the devs are supportive, or at least accepting of it.
To persist in modding a game despite the devs being vocal about hating mods, one would need a contrarian streak (like the folks still chugging along with the hgg mod for CoC1).
I haven't played that many games found here, but of the ones I've tried:
MGD supports mods and has a good amount of quality offerings available. I think at least one of them has been adopted into the main game (Gren started out as a mod, correct me if I'm wrong, please).
Lilith's Throne is currently in the midst of a dry spell without main quest progress, because the dev has been spending ages setting up all sorts of frameworks to enable easier modding (player-made races, items, etc.). LT modding is still a bit clunky, but more and more people are trying it, if only to see why the dev spent so long on it.
I haven't played Summertime Saga in a while but I had a couple mods for that which added more pubes to a bunch of the characters, among other things. One of the mods was really quite extensive in what it added. Unlike MGD/LT, I have no idea about the dev's stance towards modding.
Mods for porn games do seem to be somewhat rare though; I'd imagine most people are fine with getting their rocks off and then focusing on something else.