Another question is why don't browsers support MKV container format?
From an arbitrary search "The Matroska project is supported by a non-profit organization and is a fork of the Multimedia Container Format. It was first announced to the public at the end of 2002 and is a completely royalty-free open standard that's free for both private and commercial use."
General MKV support is approximately impossible because the subtitle format is extremely complicated and not specified. libass only implements it by being bug-compatible to an old piece of Windows software.
Also, MKV isn't a great file format; the way it does timestamps is all wrong for instance. It stores them as decimals when they're naturally rational fractions, so it's a strange choice for an archival format since it literally cannot be accurate.
Interesting about the relation between WebM and MKV found in google search:
"Many Web browsers support WebM, which is a restricted version of MKV. The most important restriction is in the allowed video codecs. WebM video uses only the VP8 and VP9 codecs, which are open and royalty-free. All WebM files are MKV files, but not all MKV files are consistent with WebM"
So the question comes back to supported codec moreso than container format.
MKV can literally contains anything includes but not limited to text based effected subtitle, image based subtitle, multi alternative audio channel, chapter separated video and even font.
And the used codec also isn't even limited. H265 10bit + flac 96khz? Fine. You can pack it as long as you want. And it's your own problem to play it. And that is basically why you see youtube-dl merges best audio + best video into MKV. Because it is the one only format that allow you to merge something like vp9+opus into playable single file.
It is basically a 'one format rule them all' format. The browser is unlikely to actually properly implement all of them. (In fact, there are only a few players actually implement all of them outside of players on windows) And even browser does want to implement everything, there isn't proper API for browser to expose these capabilities to the DOM and JavaScript to properly control them. (I think we are unlikely to see browser support native multi audio track video)
From an arbitrary search "The Matroska project is supported by a non-profit organization and is a fork of the Multimedia Container Format. It was first announced to the public at the end of 2002 and is a completely royalty-free open standard that's free for both private and commercial use."