It might be because Slack has support, a great UI, lots of users, and 'just works'. A company I worked with tried to setup Riot and Matrix and they were having

It might be because Slack has support, a great UI, lots of users, and 'just works'. A company I worked with tried to setup Riot and Matrix and they were having all sorts of problems. Even when they tried to use the Riot.im website there were bugs and features from Slack they needed that were missing. What Matrix and Riot.im is doing is still good though, I hope it takes off.