It needs to have better features and better support.

What features are you looking for? I'm sure I've seen the Mattermost/Gitlab team browse here in the past and having something actionable would help them get there

I don’t know. As someone making IT decisions, I can really only look at the options on the market and compare them. Mattermost isn’t clearly better than Slack along dimensions that matter to most teams. Its product management strategy seems to be “we have the same features as slack but you can self host”.