Slack's UI sucks for multiple different "server"s. Good for work, but no gamer wants to juggle around the Slack UI for 2/3 different game servers, not to mention DMs being isolated. Imagine DMing the same person with 2 or 3 completely separate chat histories, depending on the source server. Many people have multiple servers that share a subset of people, and talking to them would be absolutely insane, not to mention the amount of people that primarily use Discord for DMs that don't care about any server at all.