A lot of people using something like Slack is not a strong argument.
The same can be said for Crystal.
Why the language is open to the community and not just openly released, a la Android/Chromium, or proprietary?
I am not the creator of the language, but I guess they believed FOSS would have more benefits than proprietary. Manas could have perfectly make profit on this language.
It is logical to favor other projects whose people have done the same choice.
Having said that, Slack and Zulip are quite similar at the end.