Soft docs are harder and take longer, and as said elsewhere are not appreciated enough. If I'm writing an Elixir library for (what I think) my own use, I'll stop at typespecs, docstring with an example, and a short description.
I'll only start doing the tutorial doc and/or blog post if others have interest in the library. There's just not enough time :/
The thing is, it's not that there aren't a lot of blog posts. There are! There's usually at least one blog post about any library I care to use. But it's only a blog post—it's something someone wrote at some point—and so it's not corrected for misapprehensions the author (who is usually not the author of the library) had about the idiomatic way to use the library, nor is it kept up to date with changes in the library like soft docs would be.
I don't think there's a dearth of people willing to write blog posts. There's just a dearth of people willing to do the equivalent job to what Wikipedia editors do for general-purpose articles: collating those "primary sources" into a single, coherent, up-to-date overview.
I'll only start doing the tutorial doc and/or blog post if others have interest in the library. There's just not enough time :/