Youtube will let the Flash player to the past and come with HTML 5 features.
YouTube first added support for HTML5 back in 2010 when I used with Linux OS.
The main reason can be :"Most critically, HTML5 lacked support for Adaptive Bitrate (ABR) that lets us show you more videos with less buffering."
Also, HTML5 extend beyond web browsers and it's used in smart TVs and other streaming devices.
Read more about this, here.