It's not really open source project. They always gated a bunch of features, require CLA (so even if someone does contribute, boom, your code is theirs and they will probably close it down behind enterprise license if it is useful enough), and have pretty complex licensing scheme https://docs.mattermost.com/product-overview/faq-mattermost-...

> Everyone else is just going to keep using Slack, and arguably outside of these niche concerns, it's a better funded and higher quality product.

They had niche when their lite enterprise license (just basic LDAP and some other small features) was $2.5 per user.

Now they are basically on slack pricing, why would anyone bother...