It was shot on film, which makes conversion to HD and different cleanups possible. All of the others were shot on Video, so the special effects were done on Video, which makes it more difficult to upgrade them from what they are.
Because of how they were done, TNG, VOY, DS9 will all be 480P at their best. Enterprise was shot in a widescreen 720 format, but I don't see an open revolt for an HD release, though I guess it's possible. That having been said, I would love to see them re-master TNG, VOY, DS9 even if 480P was all we got, because the DVDs, especially for DS9 are TERRIBLE. They look pixelated like crazy, suffer artifacts and other issues, hell in DS9, some episodes show the black of space with big MPEG blocks. If I hadn't picked up the box set for Christmas a few years ago, I'd be pissed. The versions as they run on TV look immediately better.
But the drive for a higher quality DVD/BD release at a 480P top just doesn't seem to be in the cards. But if tomorrow someone told me they were going to release and re-encode: Buffy, Angel, TNG, DS9 I would snap them up in a heartbeat. Some shows have turned out well on DVD. Angel isn't bad; Alias actually looks decent on DVD. But DS9 looks like total crap