A few months ago I tried my hand at this because it had some better defaults. Also, I didn't like camel case variables everywhere in Hugo. Then, I gave up when I realised that Zola does not even allow YAML config files. Nit picks everywhere..
There's also a concern for maintenance: maintaining the zola codebase to support more serialization formats is more work. If more contributors get involved regularly, it will be less of a problem.
In fact, zola started supporting YAML frontmatter for compatibility reasons lately, but it's explicitly stated in the docs this is not the recommended/supported way to do things.