I've worked with two organizations now where Slack has been heavily used. There is absolutely no chance 75% of the orgs would have used IRC, even if mandated, the way Slack is used.

Even something as simple as having an avatar next to your name makes all the difference.

The real question should be what does Slack do that Hipchat did not? The designs are obviously much different, but other than that, was it just amazing marketing and word of mouth? Even the Slack founders seemed a bit baffled by this when asked months back.