- Tech debt, they can't easily add or change features anymore
- Performance, rendering syntax highlighted markdown in messages is too expensive
- Performance optimization, by not having syntax highlighting they encourage people to create snippets instead, which goes to different backends and lazily loaded clientside rendering pipelines to support larger files and syntax highlighting.
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