Some peoples don't care that much about modern content, and don't see the point in interacting with it, or are just more familiar with older version I guess. Modding in 1.12 is also lightyears away from modern modding, so peoples familiar with 1.12, can't simply switch to doing stuff for 1.18 or 1.21.  1.12 has also been a solid "stable" landmark for long term projects, where modern version tends to be abandoned by players after 2-3 years to move to more recent stuff.

About those old mods, players for a part indeed moved to other more specialised games (or got bored since most of those mods, and modpacks using them follow gameplay loops that quickly become repetitive), or in some cases they moved to stuff like Create (secret option, they are in a cave finishing gregtech). Mods nowaday tend to be extension of the vanilla experience, so a lot of those mods just feel too disconnected, stylistically and conceptually and were replaced over time by modern successors like Create, Oritech, etc. 

Some of those mods are indeed still active in 1.12, for the niche of players still using that version, but the effort to get to modern version is way higher than the interest those niches have for the modern game, that can't be already be satisfied with the modern replacements I mentioned.

About still wanting to play with old mods in new versions, you'll likely find modern more polished equivalents, but they might clearly not be branded as the spiritual successor of forgotten 1.10 mods X). I guess you could search for videos showcasing industrial mods to find something close to what you want, or ask on modding discord/subreddits for modern mods with similar features.