I strongly believe this has something to do with three things:

1.  Easy setup and Quick Invite
Unlike other messaging groups or chat apps, Discord is available for use both in app and on Web. Someone who is not comfortable in downloading another app that will take some storage space can opt out to the Web version. 

It is also literally easy to invite someone over to the chat. All you have to do is create an invite link and just leave it somewhere (perhaps your blog or site) in order for people to see.

2. Ability to create channel threads
One thing I really love about Discord is that it is so easy to create channels for specific purposes. This is quite helpful when you have many users who each have their own agenda or reason for joining the server.

Most Steem platforms prefer discord due to this. They can now easily create an #announcement, #faq, #bugs, #post-promotion, #help channel. Each with their own purposes. 

3. Open source bots and management tools