Because Ubuntu is not Fedora? (Meaning it's not an experimental, latest pre-beta bleeding edge distro, but rather intended to be solid and stable.) r600g is not considered ready. It's not even beta yet.
And yet this is the same distro that recently released their Unity netbook desktop long before it was fit for use - sluggish due to their use of the immature Mutter WM, and all but unusable from the amount of screen space the desktop consumes.