For some weird reason browser manufacturers strongly oppose adding support for any new media formats. They will only accept a new format once they decide that's a good political move. If I were Google/Mozilla I'd add support for everything ffmpeg and imagemagick support.
As soon as they support something, they can never drop support for it because some sites will have started using it, and some sites will never stop using it. Look at how hard it has been to deprecate Flash.
By enabling support, they are commiting to maintain same-or-better compatibility pretty much forever. Thats a big commitment if your code is based of someones 'for lolz' patch to ffmpeg...
If memory serves it used to be somewhat common in Unixy land before .png was added to browsers. There may have been long-lived documentation or academic pages with xbm that they were reluctant to break.
