Because it does the things well gamers want. They want to play together, and talk together (voice chat), and stream showing them bossing or doing something IG. It does those things very well. If matrix gets seemless voice calling and video/screensharing at the click of the button, they would find it easier to make inroads.

It also has sensible defaults for managing users etc. and it's effortless.

It's not about features, it's just about getting the basics right.