I’m assuming a friendly tone here, and in a similar tone its funny because I also think Nix is not adopted because its benefits just aren't worth the cost to users (devs)

I did indeed deploy Nix to moderate success in a prior gig, but have held back pushing it at my current one; we're simply not at the scale where the problems that Nix solves are worth the cost (yet, maybe ever).

For a less controversial take, consider alpine's apk package manager. For a single-use container that runs one utility in an early dockerfile stage, apk can probably produce that image in 2-3 seconds, whereas for an apt-based container it's more like 30 seconds. That may not matter in the grand scheme of things or with layer caching or whatever, but sometimes it really does.