The classic tale of web standards vs browser implementation! WebM's RIFF container structure technically supports multiple audio tracks and subtitles—it's right there in the spec sheet! But try finding a browser that actually implements this feature and you'll be staring at your screen with the same shocked Pikachu face.
It's like browsers collectively decided "nah, we'll just implement the bare minimum." Meanwhile, VLC Player is sitting in the corner supporting practically every format known to mankind since the Mesozoic era. The gap between what's technically possible and what's actually implemented is the silent scream of every web developer trying to build accessible video experiences.