I will also aim this question towards Voyager and Enterprise aswell (although in the case of Enterprise an HD release might simply come about due to the ease of time and cost to get it into that format).
TOS recieved the HD/remaster treatment and I think we can agree that was most welcomed. If the series is going to coninue to entertain us in an era where everything is high-def, crisp and clear it needed desperately to "refit"(pun intended). Of course, more importantly than us fans, are all the "possible fans" that would logically want to start their Star Trek experience with the original that spawned them all, and of course to the younger audiences of today, TOS before remastering might have seemed like a daunting task to even sit through. The point I am trying to make here is that we, the established fans, were only part of the market that the remaster project was aimed at.
Now TNG is also getting released in HD, with all glory of the remastering treatment. TNG, in many ways resonates with todays sci-fi stronger than TOS, it has around the same strength of fan support as TOS(if not more), and I am sure that is why CBS decided to go ahead with TNG: The Next Level, despite the extra time and cost it would take to get TNG remastered compared with TOS(episode for episode of course), they reasoned that the fanbase is large, devoted, and fresh enough for them to purchase the HD release and for syndication.
With DS9 though, I am not so sure. The show was never has popular as TOS or TNG, even in its prime, and at times it becomes a blacksheep amongst the core fans of Trek aswell (sometimes Voyager is viewed this way and other times its Enterprise). From what I understand, DS9 and VOY will require the same painstaking method of remaster that TNG is currently going through, so the question stands; Does DS9 command a large enough market to justify the money being forked over by CBS?
This question will be partly answered by the sales figures of TNG HD but even if TNG does well, it won't mean that DS9 would sell half as well. I guess CBS will release another sampler for DS9 at least to wage customer interest but then again sampler-sales are not a very accurate way of predicting actual product sales.