A really in-depth article on Trek News [link here] about why Star Trek: Deep Space Nine isn’t going to be available in HD any time soon has been doing the rounds.
I’ve just spent about 20 minutes trying to figure out how to summarise it, but … I really can’t, it’s too complicated. I’m afraid if you want to know all the gory details you’ll have to read the full article. However, it does provide some good technical data about how, from the 1980s up until the turn of the millennium, programs were downsampled and then VFX (and CGI) were added afterwards. Making it pretty much impossible, or at least prohibitively expensive, to make HD versions.
So, at least for now, there’ll be no blu-ray edition for the foreseeable future.