Discord is popular because a very significant amount of people prefer the casual social instant chatting aspect over the slower threaded style of forums, especially young people.
But I do see, generally young, people join niche discords that don't necessarily see 24/7 activity say something like "Hey anyone here?" then a few minutes later "lol dead server". I think it's because for a lot of younger people they mostly want the socialization aspect over just a place to share about a hobby. They go on discord when they want to talk to people.