It's simple and readable. You literally put somebody that has never coded in their life, show them the YAML file and they will probably get it. Worked both with my boss and my girlfriend.
In Toml there are too many ways to do the same thing, which I don't like. Also unless you know it deeply, you have no idea how the underlying data structure is going to look.
It's really unfortunate the devops world chose such a hot mess of a format. Extending JSON with comments would be a dumb choice and still do a better job for most config files.