Better to make 2 ops with 5 bosses than 1 with 10. Amount of content is same, and you don't have to worry about that one guy that can't raid on second day and can't find group to fit his 6/10 lockout.
And Terror is in similar(as in, not the same, similar) position to Denova and KP was to EV. With EC Nim targeted after Terror, and no knowledge if it'll come with Nightmare mode already in, or what gear will drop at what mode, we can assume those two ops are intended as pair just like EV/KP are. Same gear look only supports this idea.
Nah they said it will be a step up in gear reward too.
i.e. Storymode Terror = Campaign
HM Terror = New Set (2.2)
Nightmare EC = New Set (2.2)
Nightmare Terror = Dunno, but probably the next set after that (2.3)
It wont be launching with Nightmare, just like EC.
EC Nightmare will come AFTER Terror but hopefully soon after, and is intended as a 'Challenge' and an alternate source for the new set of gear.
Edit: I reckon the reason they are no longer making 2 'pairs' of raids like EV and KP (both Tier 1, dropping Columi in Story and Rakata in HM) is because it clearly takes them longer to create content than they claimed. And for that reason it just wouldnt make sense to release the next raid 4+ Months after Denova, and have it drop exactly the same loot.