Zensical team here. Yes, there are very good reasons for that change. First and foremost, with MkDocs using YAML, and Material for MkDocs being the main entrypoint for many of our users, we got a lot of issues with users having trouble just getting the indentation of YAML right. Secondly, and this is even more problematic: Python Markdown allows the use of custom YAML tags[1], which translate to function references during parsing. This means that YAML parsing is tied to Python and thus not portable to other languages. It's also the reason why we currently need to go through Python to parse MkDocs configuration and render Markdown. TOML on the other hand doesn't have such magic, making it portable.

[1]: https://pyyaml.org/wiki/PyYAMLDocumentation#yaml-tags-and-py...