It's the combination of persistent text chat / community driven features + voice chat / streaming / etc. that all work well together and are tightly integrated. It's easy for someone even with minimal experience to manage Discord servers, even for a large community. Roles and permissions are a breeze. It's easy to add bots that provide a ton of additional functionality. The interface is pleasant for end users.
You would need multiple open source projects running to replace all of the core functionality people use Discord for, and they would be in different interfaces, often less user-friendly ones.
And I say this as someone that ran Mumble and TeamSpeak servers for years.