The buttons you talk about are part of the bundled markdown extension. Our goal with those bundled extensions isn't to be completely feature complete, but to provide an entrypoint for use and give practical educational resources for developers to extend upon.
Remember that the Flarum team is small, the larger the codebase under their responsibility the bigger the burden. This causes development on Flarum to disperse its focus from enabling developers to build awesome extensions to maintaining extensions.
Regardless of the above, any Pull Request that adds value to a bundled extension (similarly to core) will obviously be reviewed and merged if acceptable.
