Some combination of

- homebrew maintainers wouldn't be interested in accepting this as a PR unless it's actually maintainable, as they're the ones who would have to take responsibility for maintaining it going forward. And it needs to integrate well with the existing architecture, be well-designed and well-documented, have a migration plan and fallbacks for edge cases that are currently supported but won't work with this, etc. OPs implementation works, but is some combination of hacked together / slopcoded and it would take a lot of work to get it into a shape that could be accepted upstream. OP isn't interested in (or isn't capable of) putting in that work.

- "I made zerobrew, with 7.6K GitHub stars and thousands of users" sounds more impressive than "I made a contribution to homebrew that sped up package installation by up to 100x" despite the latter being both harder and more useful.