Because those distros have (as we are talking about) distanced themselves from the patent encumbered codecs? When Google tried to get behind VP8, MPEG LA was right there to try and stop them by trying to get them into the pool.

Edit: I should have said many didn’t Fedora opted out of compiling mesa with hardware accell, and it seems others did too recently. But that means it was there the whole time. I guess most distros, that have any money, are going to want shelter from lawsuits.