In every other Star Trek plot involving time travel, the ultimate goal of our heroes is to restore the old time line at all costs, including moral dilemmas like people who shouldn't live and so on.
I know the official explanation is that the 2009 movie is an alternate timeline and the prime timeline does still exist (to calm down the fandom).
But what I don't understand is:
- In-universe, why in this case did time travel create an alternate timeline parallel to the old one instead of overwriting it (like in all other instances of time travel in Trek I remember)? There seem to be no special circumstances this time to me.
- How does Spock know that this is the case and why doesn't he try to restore the old timeline (like he and others do in every other instance of Trek time travel)?
- If Spock knows and wasn't surprised, why didn't Nero know it? And if he knew it, why did he punish the innocent people of this new timeline?
- Out-of-universe, why didn't the creators insert some technobabble from Spock to explain it to the audience and make it clear. Something like "I checked the phase signature of this universe and it seems that we created an alternate timeline" or something like that?