Additionally, as far as I know, after 1.12, the development of minecraft paced up and a lot of internals of minecraft changed. This turns the update of mods past 1.12 into a complete rewrite most of the times, which has burned out quite a few authors.
This is correct yeah, there's a lot of effort involved in porting mods to newer versions. Larger mods slowly move in that direction, but at the end of the day even now people are playing modpacks from 1.12 because there simply aren't equivalent ones in newer versions (with the same set / feel of mods)
