Nix also gives you (not me, just in case you are going to assume that) a headache, which is why it's not getting the adoption that everything else does get. Nix is the betamax of state managers.
Docker gives you the headache of not actually giving you reproducibility. If you haven't been bitten by it then I don't think you have tried.
For popularity, I wouldn't actually mind if nix weren't all that popular, but it may just have the largest number of packages available for install for any package manager.
I haven't heard of anything getting a Docker-related headache because it delegates reproducibility to whatever tools you pick. We haven't been bitten by it, and you might think I have not tried, but you'd be wrong and making some odd assumptions.
Perhaps I can assume that you have only used nix personally or at a small company?
