SEINFELD was able to spare the expense of re-editing all of their episodes in order to release them in HD (the Blu-Rays have yet to emerge, but they're already being aired in HD on television).
It's true, STAR TREK: THE NEXT GENERATION probably has a smaller audience than SEINFELD (it had about half the Nielsen audience during its run), but it still had a big audience.
The release on Blu-Ray by CBS Paramount is not a question of if, but a question of when. It may be a few years, but it's pretty unlikely that CBS Paramount will forgo the revenue opportunities of converting the most popular series in the STAR TREK franchise to HD. The further spin-offs, which feature much more elaborate visual effects and had decreasing audiences, are a much more open question when it comes to HD versions. Only ENTERPRISE is assured one, since it already exists in HD (albeit with up-converted visual effects).