Discord won because the competition sucked. What are the alternatives? Let's break it down:
XMPP - Stuck in the past, lacks modern basic features like "If I leave my laptop open and receive a message, it doesn't disappear into the ether and I can still read it on my phone". And if you do manage to configure it to match modern messaging services, you just spent hours in config-file land, which 99% of users aren't interested in attempting.
Hipchat, Slack - So now at work, when you present in meetings, and show your Slack window, everyone will see that you're in "Dave's Sexy Game Server" as well as BigSeriousCorp? No thanks.
Google Hangouts - In typical Google fashion, they've taken the perpetual-MVP approach to this product. Somewhere, someone inside Google is arguing that removing chat functionality from their chat platform is the best way to simplify the product and cut costs. Oh okay. I'll describe it. It's a 200px by 200px window of sadness with no searchable chat history.
I don't think any of those were alternatives for gaming, which is what it was (still is?) marketed for.
If you wanted to play a game and do voice chat with a group of friends, TeamSpeak, Discord, and Mumble were the only options that were group-first (easier to do lobbies than single-person calls). Discord's UI is 2 decades ahead of Mumble / TS, and it's free.
XMPP - Stuck in the past, lacks modern basic features like "If I leave my laptop open and receive a message, it doesn't disappear into the ether and I can still read it on my phone". And if you do manage to configure it to match modern messaging services, you just spent hours in config-file land, which 99% of users aren't interested in attempting.
Hipchat, Slack - So now at work, when you present in meetings, and show your Slack window, everyone will see that you're in "Dave's Sexy Game Server" as well as BigSeriousCorp? No thanks.
Google Hangouts - In typical Google fashion, they've taken the perpetual-MVP approach to this product. Somewhere, someone inside Google is arguing that removing chat functionality from their chat platform is the best way to simplify the product and cut costs. Oh okay. I'll describe it. It's a 200px by 200px window of sadness with no searchable chat history.
Whatsapp, Facebook Messenger - Mobile-first experiences.
Forums (PhpBB etc.) - Better for offline communication, mobile sucks, hard to keep track of new messages to conversations.