Allow me to shout into the void for a minute here...
1. Slack is a well-designed interface for allowing teams to communicate via chat.
2. Slack is easy to install is use on Mac, PC, iOS, and Android. It Just Works™.
3. Slack doesn't require me to install IRC somewhere. Which also means I don't have to worry about how people gain connectivity to said server when outside the office.
4. Slack has whimsy. Fun colors, messaging, emoticon, bots, etc.
All of this is what folks in this thread seem to be missing. I've used IRC for a very long time, but have NEVER been successful at getting wide adoption of IRC for communication.
I am well aware that I'm trusting a third party with out information. I'm aware that alternatives exist and you can get them to work. That doesn't matter when I have to try to explain to my CEO how to /join #channel.
There is a reason why IRC, a widely-available, chat solution that has been available for decades didn't catch on. It has nothing to do with how well the software moves messages from one computer to another.
</rant>