> Tech debt, they can't easily add or change features anymore

It's this. I've worked on Slack applications and you can clearly see why they might struggle to implement this given the data structure of their messages. Their brand of markup is also quirky and it might be difficult to add this feature in a backwards compatible way

Snippets was added much later and part of me feels like the demand for syntax highlighting was exactly why they added snippets. It's also a scapegoat for them not adding syntax highlighting to regular messaging