> What stops fiction authors from picking a niche as well?
Trad publishing and a lack of any real discoverability.
I don't read much fiction in text format, anymore, so take Audible, for example. I have a backlog of maybe 50+ books in that format, and the store front is atrociously bad. There is a lot of what I call "pulp fiction" pushed at me - some of it is OK, most of it is garbage, good in the way that stale rice will feed you and not kill you, but only barely edible.
You are given a selection of maybe, 9-10 books, if you want to "discover" you have to do quite a bit of work, which the trad publishers don't make easy (remember, marketing budgets push particular books, not good books).
So even finding a niche is close-to-impossible for the average Joe - trad publishing pushes what sells, despite quality, and so niches simply don't form.
So you can imagine, those who are self-publishing, must do a great deal of effort to even establish that niche (website, marketing, distribution, technical knowledge for all the former), and, unlike tech authors, they aren't usually tech savy.
So I would say two things stop them in particular:
a) There is no access to manuscripts which is not filtered by nepotism, or a small crew of editors. You can't self publish and submit to github, and expect a niche following to occur.
b) Authors are human beings, who want to make a living. Understandably, they don't want to give away their hard work for free (their manuscripts), and we don't have a commonly agreed upon way to benefit them for their work, if the public decides they are good.
Tech has less of a problem here, we often work backwards. We work on something for free, give it away on github, find it gains a niche following, and experience some sort of reward down the line (hired by major tech company, doing consulting work, e.g.). We also don't have the same market forces - nearly everyone reads, almost everyone can write (well is another matter), we can solve esoteric problems, fiction writing is often formulaic and thereby constrained in topics.
We still experience the forces of trad publishers in the form of app-stores, OS or hardware vendors, but we aren't as limited as fiction writers are in the scope of our domain. Tech writing is also usually extremely ephemeral, often a just published tech book will be out of date as soon as it is available, or one or two years into its existence. Not to say that tech writing is not useful or work, just illustrating how different the problem spaces are.