The videos used on the micro:bit site were produced by the BBC as part of the original Make It Digital Programme, and these were encoded using the Flash format at the time.
The microbit.org website just links to those BBC provided videos.
We have checked with the BBC for ongoing policy on use of Flash vs HTML5 video, and while they don't have a specific public facing page about the ongoing migration, the links below were kindly provided to us by the BBC to explain the ongoing migration process of existing and new content. They inform us that the process of re-encoding all existing BBC content is time consuming, but it is ongoing.
At The Micro:Bit Educational Foundation, we are committed to using non-Flash videos for new content that we develop in the future.
BBC 31/01/2017 (JT)