Having used both at the time, it was all about UX and marketing. Back in roughly 2015-2016, some of friends were using Slack at their company and were making f

Having used both at the time, it was all about UX and marketing. Back in roughly 2015-2016, some of friends were using Slack at their company and were making fun of how antiquated and uncool HipChat was — even though HipChat had all the same functionality. HipChat had the advantage of being a fast, native app, while Slack was a slow Electron wrapper around a web page. But Slack nailed the UX. It had the aesthetics of a generation of "post-Web 2.0" apps that were coming out that were more real-time and shipping as Electron apps. HipChat just looked old school. Pretty soon my company was on Slack, too. It also helped that Slack quickly got support for a much richer set of integrations with other apps. HipChat had a bunch, but I think Slack realized this would be the number one way to beat the competition, and they spent a ton of effort on building out this side of the platform. Slack, of course, also had the advantage of focus; HipChat was made by Atlassian, who had a lot of different products to spread their focus more thinly. Around the same time we saw a similar battle between Google Hangouts and "old school" apps like WebEx and GoToMeeting. The latter were/are native apps, rock stable, but fundamentally uncool. Hangouts was browser-based and super flaky, but somehow cool because they were the latest generation of web UX. What's interesting right now is that Zoom, a thoroughly clunky and uncool native app (very similar to WebEx or GoToMeeting in that sense) has somehow gained enormous market share, despite the existence of decently mature web-based alternatives. Maybe because Google managed to mess up its chat/videoconferencing so badly.