Saw this on another board as a possible explanation for why Spock doesn't just time travel to fix things:
Well, he can't go back into the future at that point to fix things as he's already in the alternate timeline. And if he goes back in time to when Nero first shows up, how exactly is he going to stop him? Nothing in that time era is capable of destroying Nero's ship, and he has no access to the red matter in his own ship as Nero has it in his possession.
Makes sense to me - if the Narada can lay waste to anything from that time period, why would Spock try to go back in time to stop it when he has nothing that would help?