There are 102,500 members in the Slack right now (though not nearly all are active), and Hack Club is mainly focused on getting teens interested in coding. It needs to be approachable for non-technical teenagers. Also, as someone else said, we build many integrations around Slack, like how users update their password and SSH keys on a VPS through a Slack bot.
Hack Club had a $5,000/year contract with Slack (renewed in May iirc), but Salesforce just suddenly told them to pay $50,000 within a week and $200,000/year, without warning, or they would deactivate the whole workspace. That's how the HC founder told it in the Slack announcement, anyways.
Another Hack Club member here, this situation is hard on many of us since we built many of our projects around Slack integration, and we now have to rapidly re-code them so they don't break. It's not great, especially in the middle of the school week (reminder that hack club is a coding nonprofit for teenagers, so i have to go to school and have homework while doing this)