Compared to yaml or ini formats, JSON is arguably harder to read and definitely harder to write due to having to keep track of brackets, commas, and/or quotes.
Ideally, a configuration file should be easy to read and write/modify by a user of the application. A lot of applications just stick with ini like formats because it meets both requirements.
> I really struggle to write yaml. I end up writing it as json and converting it.
Since valid JSON is also valid YAML with the same semantics, it is impossible for it to be harder to write YAML than JSON, and no conversion is necessary.
Ideally, a configuration file should be easy to read and write/modify by a user of the application. A lot of applications just stick with ini like formats because it meets both requirements.