Because without that hope and actual addition, GC3 is basically a GC2 expansion, gameplay-wise. Differently named research and hexes don't bring a fundamental difference.
Um, I think you are underselling the additions between GC2 and GC3. Besides, shouldn't a GalCiv game feel like a, well, GalCiv game? IMO too many game studios try to change things for the sake of change. I want GC III to feel like GC II, personally. Otherwise why call it Galactic Civilizations?
More than that, the gamedevs on more than one occasion have said that this IS like GCII, but a deeper, richer, more complex experience. And, personally, I applaud them for that stance.
EDITED::: Funnily enough, I had reason to fire up GCII last night. It's surprising just how different it looks even without getting into the numerous game mechanic changes. Personally I think the memory might be cheating a bit. Go up and load Twilight and compare it to the beta as it stands right now, and it is clear that these are different games with similar but different mechanics.