IRC is distracting!
MSN/Skype/etc. are all IM-style clients, so you have to actually select a person to talk to rather than just dropping an @everyone. They vaguely support group chat but it's not the default. I would like Slack a lot more if they emphasized individual and ad-hoc group/topic chats. They support them, of course, but large channels are the main focus and the UI steers you there.
Lync/Skype had a persistent chat room product/extension that you could essentially use like Slack.
I'm not sure anybody really did, but you could...
We do. It's awful, but irreplaceable.
Actually, the terrible UI may well be a benefit. I've worked in a company with heavy investment in Slack, too, and its ease of use I think contributed heavily to the low S:N ratio of channels with high population. Lync's group chat is so painful to use that, unless it's important, we don't.
I'm no big IRC user, but when I joined some of the open source IRC channels, there was activity all the time; how is that less distracting?
Also, do not disturb and 'quit' are always an option.