DRM. Until HTML5 standardizes DRM across browsers, these sites relying on DRM won't be getting rid of Silverlight and Flash.

Incidentally, Netflix has started using HTML5 instead of Silverlight/Flash, but only on Internet Explorer 11 on Windows 8.1, since HTML5 DRM extensions are available on that platform: http://www.extremetech.com/computing/159960-netflix-switches...