In fact it would have been seen as a sacrilege to use any other medium for videos.
Also technology wise things are different now.
Also with new technologies came new browsers which were capable of supporting different formats without a hitch.
With the upgradation can the inherent change and YouTubes move to HTML5 is one of them.
To move from the Flash based videos to HTML5 support YouTube had to embark on a massive technological exercise.
First of all they have moved to MediaSource Extensions to provide Adaptive Bitrate (ABR) streaming.
VP9 allows allows more people to access 4K and HD at 60FPS and videos start 15-80 percent faster.
YouTube hopes to make VP9 codec mainstream in the future.
Third for encryption, YouTube has taken WebRTC support.
source: www.techworm.net
