It could've been the HipChat revolution. It was so close to being just that. As a user of both HipChat and Slack, however, the difference is vast. Slack had a rich API an integrations well before HipChat did; presents rich content substantially more attractively; had first-class mobile support off the bat; tracks unread messages when you're offline (this one is extremely frustrating in HipChat); made being a member of multiple teams simple; had a much smoother signup process... etc.

The organisation I work for got on board the HipChat train just as Slack was gaining momentum. While it's cheaper, I think if we'd had Slack we'd have much greater adoption.