Maybe because it is (or at least can be, the more advanced usages become less so) tolerably self-describing in that you can readily see what it is producing, in a way that SVG isn't, while still being source-control and diffing-tool friendly text?
For small things like its intended uses, it fits well within coding tools and workflows, and avoids separately documenting and coding the art.
This kind of reeks of "If your only tool is a text editor, every problem looks like text".
Just as with programming, polyglot programming has value, but switching languages within a project had a complexity cost that isn't always justified even when for dinner task taken alone a different language is marginally superior.
For some applications, having something be both can be helpful.