Archival as in has to support decades of old images. It's not a format that's being heavily modified or is going to see any hard transitions in coding/decoding.
FLIF and BPG are development formats and where you look towards the future, not the past.
Sure there are JS polyfills but those aren't going to work well on mobile devices and until content creation tools like Photoshop and desktop operating systems like Windows and Mac support exporting and reading BPG, it's unlikely to see much adoption.