The simple answer is the browser as a platform stalled for ten years or so (the "Flash Era"). It's only very recently that real work was put into adding new ca

The simple answer is the browser as a platform stalled for ten years or so (the "Flash Era"). It's only very recently that real work was put into adding new capabilities like video playback (See the WHATWG conflict). Beyond that, I would add that the natural complexity of video encoding and the legalities which surround it complicate things. The video playback implementation bundled into the browsers is simpler than what you might find in something like VLC, making it a bit less capable of handling all the potential oddities a video can have. You have to actually encode your video in multiple formats, and even then be very careful, to get it to actually play in every browser at the moment. Hopefully these things will stabilize in the near future, but it's early days.