I like Discord well enough
for the right purposes but it does make me truly sad to see so many niche communities go behind closed doors like Discord.
Discord isn't indexed or searchable by folks (or search engines) outside of the server, and even then, it's a real-time chat application that just doesn't lend itself to the type of content that makes reddit special. Discord also has investors who will want payback on their investment...so users on Discord may find themselves in the same boat some years down the line.
But people use it because the barrier to entry is so low. Even non-technical people can set up good looking and functional communities.