Probably because they don't want to (or can't) have a huge portion of their company's internal communication stored forever on a third-party server.

I still find it hard to believe that so many companies are okay with doing that.

And yet most large companies are using AWS and the like. This obsession with "MUST be behind OUR firewall!" is very arrogant, penny-wise pound-foolish thinking - and it tends to wither in the face of a frank, honest cost/benefit analysis.

Amazon probably has more, if not better, security and engineers than most of the companies we're talking about.

Pricing mostly Can't host it ourselves, not really an issue now that we have a new boss and he doesn't care as much about hosting everything ourselves. Thats why moving to stride was in the works. I tried testing slack with a sub section of our team and they didn't like it at all.

I've been toying switching to mattermost for a year now though. See how it goes.