Cause it's very verbose. A lot more syntax to break.
I personally think the best is one of the humanized json ones like https://maml.dev/
It wouldn't need to be if closing tags were allowed to be unnamed. For most cases, we can tell the closing tags easily enough for simpler files:
<project>MAML</> <tags> <item>minimal</> <item>readable</> </> <!-- A simple nested object --> <spec> <version>1</> <author>Anton Medvedev</> </> <!-- Array of objects --> <examples> <item> <name>JSON</> <born>2001</> </> <item> <name>MAML</> <born>2025</> </> </> <notes> This is a multiline raw strings. Keeps formatting as-is. </>
It wouldn't need to be if closing tags were allowed to be unnamed. For most cases, we can tell the closing tags easily enough for simpler files: