To me that's what other technologies like rkt seem to miss or at least choose not to focus on.
They're not much different from nix expressions, or brew formulas or such.
Unlike nix expressions (or rpm specfiles or various other technologies), the dockerfile format never concerned itself with clearly defining the hashes of inputs and dependencies, reproducibility, working without network access on build farms, etc etc,
Sure, it ended up being popular and usable, but at the cost of taking the art of describing how software is installed back in time.
Could it be that these things are mutually exclusive?
Anecdotally, I've tried to learn Nix for several years. I learned to write Dockerfiles in hours.
Don't confuse expedience with usability.
Out of interest, what did you find difficult? I ask because I spent last weekend setting up NixOS, and the only things I found difficult were discoverability and documentation (I spent a lot of time just reading through the source code).
Edit: Thinking about it it's very possible that this is just a case of me not knowing what I don't know, since I'm not currently trying to do anything particularly complex.
The language is not too bad, but how it should be used is the missing part. Like a cookbook or a properly populated Stack Overflow site.
When I was on Linux, I liked to distro hop few times a year. Whenever I got back to NixOS, I felt that I had to relearn everything, and that there was nothing to grasp on to. I always take that as a bad smell.