Hacker News:
is closed source
is a walled garden
requires a "Y Combinator" account to communicate
Most people don't use software because of the driving philosophy behind it — they use it because it solves a problem of theirs.
If a tool is the best one for the job, you should use it. And if Slack has a substantially better UI for communication, maybe developers of IRC clients should try to build a better UI rather than complain about adoption of Slack.