Discord was something that the internet needed, a group chat with decent security and not difficulty to mess around, contrary to IRC, which was already dead for years before Discord came out anyways. Then there's also all the instant messengers being dead/dying for some reason or other, like MS who tried to force people to use Skype by killing MSN, which instant backfired since no one likes Skype for anything besides real business stuff, things like TeamSpeak and the likes was a much worse circlejerk and used mostly as a voice chat for gaming and people instant messenging over smartphones now that you didn't need to pay for SMS anymore.

Discord just came at the right time, is easily accessible by browser and have customization that old chats lacked plus it's all the tools into one thing, you can voice-chat, stream, chat 1-1, chat in group, etc. And lets not forget the main attraction, which are the custom emoticons/Nitro.

In hindsight, it's pretty crazy Discord was able to get its foot in the door and grow as fast as it did given the time it came out.