MKV is simply a CONTAINER, not a file type. MKV allows for multiple language files and multiple subtitles to be included in one CONTAINER so that only one release can serve many languages, eliminating the need for a file for each language to be uploaded and stored and made and shared.

That is the main reason and a secondary reason is they were simply waiting for stand alone devices such as BluRay players and Android boxes to have the necessary software to play the CONTAINER. Almost all MKV CONTAINERS contain an mp4 and an audio track and subtitles, whether CC or a language other than English. This will now make it much simpler for the people who encode files like I do to only have to make MKV format encodes. VLC is a much superior player than Windows Media player and can handle 99% of what ever is thrown at it. Hope this helps you to understand a little better. Now the smaller version of files can support multiple languages and subtitles in one container. It is being done for you the end user. Read more on below links…

The SD TV x264 Releasing Standards 2016

 and

TV RELEASE GROUPS SWAP .MP4 FOR .MKV