Because they were seemingly the first ones to a) get that combining these features together in a way which is easy to use and administrate was valuable and b) i

Because they were seemingly the first ones to a) get that combining these features together in a way which is easy to use and administrate was valuable and b) implement them somewhat competently. Most of IRC and the various other alternatives commonly mentioned are stuck on A even just for plain text chat before you get to multimedia, and most would-be competition that gets A either fails at B or got there too late. Privacy concerns are so low down the priority of users that there's pretty slim chance it will be enough to sink discord's first-mover advantage.