Most of them are mp4.
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 to only have to make MKV format encodes. Now the smaller version of files can support multiple languages and subtitles in one container. It is being done for you the end user. Hope this helps you to understand a little better.
In short: .avi used to be the standard for scene releases. The scene adopted .mp4. .mp4 is still the standard (until hevc is adopted). The mkv container was adopted as a standard holding container for the standard file(.mp4) to make it more convenient for the release groups and better for the end user.
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