JSON & CSV, whilst undoubtedly simpler formats, still lack the 'power' of the XML ecosystem.
Powerful tools to create a schema which can be used to easily view info about data/validate data etc..
Interesting to see people try to use JSON for config aswell. That's never going to end well.
The case for XML as service payload IMHO is when you need an upfront design with schemas and all for multi-party or vertical data exchange (such as for payment providers to stick to TFA's topic).
"Power" is often a compromise. There are convincing arguments that plain text is, after all, more powerful than JSON, CSV and XML. It all depends on the context where this power will be exerted.
If you only need to store name-value pairs, it may be better (e.g., more robust, more powerful) to use
scanf("%s=%d", &name, &value);
Unfortunately now I wanted to store a multi line value and the library I am using does not support that. I need an entire new config system
Some people are trying to use YAML for that too.