They probably thought it would be easier to takeover rubygems than ensure every dev and every machine for every possible ruby tool could be and is pointed at the internal gem repository.

Let's be paranoid for a moment. What if there's a supply side attack on a gem used by Homebrew. That's basically installed on every dev machine, auto-updates automatically/silently, could have sudo, that no one would care or even know how to point at a private gem repository.

It was my understanding that they wanted to use Nix to solve this problem.