As has been said in other threads (check the TOS forum), TOS was shot AND EDITED on actual film, so you could theoretically watch them on an actual film projector in a movie theater and they would still look good.
Non-remastered TOS on Blu was more due to fan demand than ultimate image quality - that's why they did TOS:R in the first place, because they originally decided that the old FX wouldn't look that good at such high res.
The key here is "and edited". The modern series were shot on film but only exist in their final forms on video, and even at that, non-HD video, as 400-odd line LaserDisc/SVHS was about as good as it got 10 to 15 years ago. For them to look acceptable on BluRay, each series - TNG, DS9 and VOY would essentially have to be put back into post production, every episode's edits duplicated from the ground up, and all the FX, if not at least re-composited with current equipment, then re-created digitally a la TOS:R. TOS:R was 3 seasons, T/D/V total 21 seasons - that's seven times the amount of FX work, plus all the editing.
If it happens at all, it won't be any time soon.