All Trek TV series (other than Enterprise
) must be remastered to make them airable in HD.
For TOS, it's because the original effects were badly degraded (and, in some instances, simply suck).
For TNG, DS9 and Voyager: All effects (and editing) were done on videotape, which - by definition - is not airable in HD, since the native resolution of videotape is so low.
For TAS: Just take a wild guess.
Would you want animation that looks that bad
to show up anywhere near HD? A full-out CGI remake of the show would be a sight to behold. And they could even use the same voice tracks.
is the only Trek series made from the get-go to be airable in HD.