It seems word of mouth has been increasing (I've been using Linux since 2014 and only heard about it in late 2020). Maybe
- it has to do with DLTs like Cardano adopting Nix.
- influencers like youtubers and podcasters have began talking about it
- migration from IRC to Matrix, so people can discover it more easily
- people who are into functional programming cycles often talk about it, e.g. haskell, lisp, emacs...
- with the raise of Flatpaks and Snaps, people have been talking more often about Nix as an alternative. The same can be said about Docker/containers and is often suggested as alternative
- mainstream Linux distros taking inspiration from NixOS to make their own immutable and atomic OSs: Fedora Silverblue, openSUSE MicroOS, Clear Linux, ...
- since mid 2010s there was some push towards making documentation better for beginners. The official website also got a modern look to be more attractive. So also taking into account nixpkgs is probably good enough for the average user and has a good enough ecosystem around it as well, this all means NixOS has become mature enough for willing/passionate adopters (e.g. Arch users who want the next shiny tech), and who will talk about it in every given opportunity.