Slack is non-optimal for open source projects because each project will require you to have yet another Slack account. It also isn't IRC-like. With Gitter, you

Slack is non-optimal for open source projects because each project will require you to have yet another Slack account. It also isn't IRC-like. With Gitter, you can be on multiple projects at once in one client UI. So different use cases mean a different product might be better. Particularly if business concerns limit how one can use existing products.