As "ridiculous" as it might be, we're a small non-profit and have to prioritize where to allocate our resources as we develop Zig. When it comes to macOS, we follow the same policy as Apple: support the latest 3 versions of it.
Maybe once Zig is fully developed we'll focus our effort on more retrocompatibility.
If you want to help us get there faster, consider donating to the Zig software foundation, as we're looking to hire more developers to work full time on Zig (we're 4 full-time people right now).