DS9 will always be the toughest sell to networks. Heavy serialized shows have a very hard time getting decent ratings in rerun syndication. While DS9 might be a more richer experience for viewers it requires the most commitment to follow. But viewers tend to get fed up waiting every week for the next episode, while episodes of TOS or TNG can be seen entirely out of order with no problems. Trek was born to be episodic, the reruns of TOS was what kept the franchise alive for decades until we got new material.
I can see CBS doing VOY before DS9.
I disagree, VOY would be a bigger, more complicated and more expensive project at this point as it would require a lot more CGI input than DS9. Also DS9 is at least as popular as VOY if not more so. And a serialised show is more modern and can lure people in for longer more easily as people want to see the resolution of the story lines that they have already invested so much time in. Also DS9 isn't heavily serialised as you put it. Most of it's episodes are actually stand alone. In the end I would like to see both in HD. But I don't think DS9 compares unfavourably with VOY when it comes to finding a rationale for remastering.
In the end if they come to the conclusion they can make a decent profit then they will do it. TNG remastered is apparently doing better then they had ever hoped it would. So there must be a margin there for the less popular shows DS9 and VOY. Also with the expertise they gainged from remastering TNG and reduced costs from improved technology etc. remasterting these two shows might be cheaper than TNG in the end making it even more likely it will happen.