The need for new HD CG would be the stumbling block. Everything from Odo's morphs in Emissary
to the epic battles in What You Leave Behind
probably don't exist in HD resolution, and would need to be redone from scratch.
As much as I'd offer up my first-born to see the wormhole in HD, I don't think CBS would see enough of an audience for DS9 to spend that much time and money.