Because he recreates the experience of an average user who comes into nix.
To quote,
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nix-env invocations are too slow for anyone to reasonably use Nix as a package manager. Full stop. Every. Single. Command. 30 seconds. It was unbearable. I mean, it was nice for writing this blog post – plenty of time to get my thoughts down – but if I were an actual, normal user? Trying to use Nix? I would just stop. My Nix adventure would end before I finished running the first command. Because I would assume it was broken, ctrl-C outta there, and go back to using Homebrew (or pacman, or apt, or whatever).
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Next I want to write, "For me, I enjoyed it," but I hesitated initially, because I'm surprised that I'm not more annoyed.