Man this is a broad question isnt it? You can easily explain why the modding community is so popular just by explaining the history of the elder scrolls, and the history of modding culture, plus how TES once upon a time was the poster boy for modding.
Or, explain who both communities, TES fans, and Modders, ended up in the same ecosystems, one that has become diluted over the years by drama, egos, and media attention. Making some of those old school modders look for new communities to apply their hobbies, and how adult skyrim mods are sorta the best of both modern, and older modding combined into one.
Or we could also delve into the sociology and popularity that comes from a massive AAA game with very accessible companion software that can make just about anyone feel like a developer regardless of skill level due to how easy the software is to use, plus the endless amount of tutorials, and communities you can find who is willing to help.
Perhaps talk about websites like Kotaku, or youtubers like MXR who showcase those mods for millions to see, vs say Milfy City that doesn't get that much praise (although MXR sorta did two episodes with it)
Or we can rant about the faults of porn games in general. About how the general public would rather just watch porn that click 200 times to jerk off, often without sound, which for many people is a huge turn off. In many ways porn games aren't worth it.
Skyrim is generally a fun game to play. With or without mods, with or without adult content. Most adult games simply lack that appeal. Unless we count some games from Japan, than the majority of adult games are either visual novels, or endless walls of text. Which to the general population would bore them quickly. Some games to offer gameplay elements, which would attract more people, sadly those games almost seem to discourage those already in the communities who prioritize visual novels. Why would a dev spend so much time fine tuning game play when the community wont care anyway?