I guess because someone messed up big.
Sounds like it. I can't imagine why they would forget about all of that from their most recent release.
And the menu on the rear of the slipcase clearly indicates that the 'Additional Materials' disc has the features from the original R1 release. Mistakes do happen. And actually, I've been so pleased with everything else about these, that the overall experience is tremendously positive.
That said, I watched TMWTGG last eve, and found it a bit less spectacular than the others in the package. Maybe my expectations were too high, maybe the SE was already of a high calibre. Definitely an upgrade, but not as mind-blowing as some of the others.
I hope I'm not overselling these, getting expectations too high. My own expectations were mostly in check, so perhaps that has something to do with my feelings about them. But I really don't think anyone would be disappointed. What K1Bond007 says a few posts up is notable, though. If one has an older 'tube' tv, one might not be able to get the full effect of the UE upgrades. I say 'might' because I no longer have a non-HD tv.