Speaking as a member of Hack Club and a former summer intern of the same, we started on IRC if memory serves. The first issue is that HC aims to serve teen coders of all skill levels, and IRC is hardly a user-friendly medium. Sure, a skilled power user can learn to work around its quirks in a few hours. However, a beginner to programming/complex computer skills with nothing aiding them but a passion for learning more would find it confusing enough that giving up before learning the ropes is a realistic possibility. In addition, we make use of message search, threads, and other rich features (think Slack Canvases, Huddles, ping groups, etc.) that either can be added to IRC or are already in some server implementations, but simply aren't powerful and user friendly enough. I hope this helps answer your question :)