Zig isn't memory safe, has regular breaking changes, and doesn't have a garbage collector.

First reason in my mind is there isn't an abundance of Zig programmers internally in Microsoft, in the job market, and in open source. It's probably a fine choice if you're using it for your passion project e.g. Hashimoto.

For being production-ready?

Zig still isn't production ready and isn't memory safe as Go, most likely.