The reason why a lot of characters do not get updated content regularly is down to few things.
1) They are not that popular. Just because you like it does not mean rest of the player base like it. Which probably leads to Savin and Co. pushing there content aside. Still, some writers enjoy writing for those characters enough that they will push through it even if most people do not care for it (See half of Bubblelord characters and his insistence on making Takahiro lovable).
2) The writer is not constant part of the writing team and they have other projects. Some authors, such as B and Gardeford just seem to write whenever they have time for so most of there characters recieve contents in short bursts. And since a lot of other writers do not have time to write for others (except if you are Observer that mad lad) they are left on the previrbial shelf on further notice. And just now, Atugias head rolled past me like tumble weed.
3) Some characters do not have that big of plans for them so they are mostly used as way to build on world and its inhabitants while also fulfiling specific sexual kink and design space that other existing characters maybe do not have reason or need to fulfill. They maybe will get something in the future but do not count on it. Look at random enemy encounters for example.
4) Artist got bored of them or left the team and no one wants to write instead of them . Now this is one I can hardly think of for examples since I do not have any of the authors to ask them directly and it rarely feels like character is just left for dead because author got bored of them. Goblin milf and most of town merchants could be argued to fall into that category. But at same time, Goblin MILF author just does not write much and merchants really do not need to have that much content around them since there primary thing is to sell loot. So yeah hard to tell.
And this last one is most important one in my opinion
5) The amount of content character gets is by the will of the writer or larger group colective (Savins choice). Characters are main content in this game. You know it, Savin and co know it, we all know it. Spreading out release of content for popular and planed NPCs not only makes us, the players , look forwoard to seeing more of there content, but it also keeps audience constantly engage. It also allows authors not to get burned out writing for same NPCs, it allows them to develop according to happenings in the story and it makes eventual release all that sweeter.
While yes, this all can lead to major case of blue balls, I think it is effective way of keeping us all engaged. How many of you are not opening this forum or there sites in hopes of seeing your favorite characters name writen in the patch notes. Until you realise it is just updated CG while Cait has gained all new content involving her new kitty prick. Writen by Observer, anal scene cowriten by Savin.