Because most people used ventrilo, teamspeak or mumble, which required running or paying for a server, and had limited chat/ social network functionality, and skype which was just terrible. It came at the right time as gaming hit its stride in the mainstream when people needed a place for 'local' communities that was basically frictionless and discord was there to capture that audience because there was basically nothing else.

All the features afterwards are mostly just them throwing stuff at the wall and seeing what sticks.