99.5% percent of people don't use Docker because they have a passion for writing dockerfiles, or because they find the ideas behind Docker elegant. They use it as a tool to help build other things. Meanwhile, the Nix community keeps pushing out these ideas about how Nix _works_ ("reproducible builds!", "lazy evaluation!") but they don't seem to particularly care about making Nix easy to use for the majority of the population out there who may want to use Nix as tool and don't care about its technical merits.
On top, I think most projects would benefit from having more functional programming advocates in them. Nix suffers from the opposite problem: it would benefit from having _less_ passionate FP people in it. Nix the language is pretty inscrutable for lots of people.