Ease of use.

With your one Discord account you join all the servers you care about and interact with online/IRL friends.

There are downsides that people at Lemmy will find dealbreaking and I absolutely agree with them, but the convenience of it being a chat, forum (almost), voice/video calling and gaming platform for free with (currently) no ads triumphs all of that for the majority of people, and that's what makes it popular. It's also cross-platform so no one's left out.