For a while we used Mattermost at my office. At the time we had a handful of startups in our incubator. Eventually we ended up switching to Slack. I don't think it had to do with anything other than momentum and popularity of Slack, in spite of Mattermost accomplishing the same thing with virtually the same interface, people just liked and wanted to be on Slack...
Slack is using cloud/external storage. Many companies have policies against that,making MM is surprisingly popular in banks and such. Personally, can't tell the difference between two
