The TOML readme has a small section comparing itself to JSON, YAML and INI:
In some ways TOML is very similar to JSON: simple, well-specified, and maps easily to ubiquitous data types. JSON is great for serializing data that will mostly be read and written by computer programs. Where TOML differs from JSON is its emphasis on being easy for humans to read and write. Comments are a good example: they serve no purpose when data is being sent from one program to another, but are very helpful in a configuration file that may be edited by hand.
The YAML format is oriented towards configuration files just like TOML. For many purposes, however, YAML is an overly complex solution. TOML aims for simplicity, a goal which is not apparent in the YAML specification: http://www.yaml.org/spec/1.2/spec.html
The INI format is also frequently used for configuration files. The format is not standardized, however, and usually does not handle more than one or two levels of nesting.
From: GitHub - toml-lang/toml: Tom's Obvious, Minimal Language
While I can not speak for the Rust authors, having used the different formats in my own projects (as well as JSON variants such as HCL) I believe they made the right choice.