They used the tool that was available at that time. I am sure they use internal chat apps as well in today's environment.
I really don't want to try to promote Slack as 'one tool to rule them all' or advocate for its features, but it definitely more bandwidth than email. Not sure have you received any of the long quoted emails recently, I have, and it can be a nightmare (and ridiculous that an email client from a USD 3 trillion dollar company cannot render it properly).
Given that Slack has integrations with various tools (incident reporting, various bots, feed submissions, apps of all sorts), video/voice chats, file storage, rich messages, advanced notifications, and, most importantly, seamless communications with clients using it, it is just a tool that has replaced so many different tools.
Sure, it is not perfect, and many other tools offer same things as Slack, this pricing situation is ridiculous, but there is a reason why nearly every single startup or a team formed in the last decade uses it or its equivalent.
It is not indented to cover all possible usages out there, and in academia I could see email working better than Slack, but as we are on the topic of Hack Club, it would be hard to argue it would exist in this form without Slack-like tools.