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

Every dapps on steem has their own demographic. For example dtube has a demograohic that always posts on dtube and usually those posts are videos. And as days pass these apps keep growing. Musing has one the same too. why is that? the answer is-

Think of a near future where steem has acquired more then 10 million users.. what happens then? the founders of those apps always need to hear what their demographic is saying about their service. does it need tweeks or change somewhere? if they have to find that out themselves on their own then steem doesn't remain truly decentralized.

So to avoid it what can they do as they can not use a dedicated chat based on steem. they use the next best substitute which has enough functionality and features needed. And there is one. Discord. 

If the app creators wants to expand and grow they only can do it with their users. And communique is key in that factor. that's why they use discord.. some of them use telegram too. but discord has the ahold of the majority.