It is not hard, but it can be time consuming. mod authors discontinue their mods for:is it hard to update a mod? :O
what happen if a mod autor won't update? i know thorium/antiaris/calamity and big major content will but ( i really want dead spirit mod to be updated and some smaller like wireless / gamble npc ) can't play without it )
working on other mod (like the author of tremer),
quit terraria (most likely what happened to crystilium),
considers mod complete,
considers coding hard,
says they will come back but they don't,
physicly unable to continue,
quit terraria community,
author dead (in theory),
lack of time,
possibly other reasons.
You can even fall under multiple things on this list.