The problem with XML is that most tools around 2000-2010 used it for everything, including simple configuration files and even DSLs (the Maven v1 XML was a nightmare to maintain).
However, there are features of XML that are very useful for a document format like SVG. One of them is namespaces. For example, you can create an SVG and add custom metadata in another namespace, and then any viewer who uses a decent XML parser will show it without issues. That’s not possible with JSON, as there is no guarantee that an unknown key will pass. (although I prefer a simple format like JSON or TOML for config files)
However, there are features of XML that are very useful for a document format like SVG. One of them is namespaces. For example, you can create an SVG and add custom metadata in another namespace, and then any viewer who uses a decent XML parser will show it without issues. That’s not possible with JSON, as there is no guarantee that an unknown key will pass. (although I prefer a simple format like JSON or TOML for config files)