It's because time travel doesn't actually occur. You don't travel within your own timeline; you
can't (because of the grandfather paradox). You can, however, travel to other parallel universes, which will just happen to be at various points in their own histories (not necessarily the exact same time and date as your own reality when you left it).
Because there are an infinite number of parallel realities, you can travel to any point in time in someone's reality, which may seem to resemble that point in time in your own reality. This creates the
illusion of time travel that has got everyone fooled.
Nobody in
Star Trek ever time travels, not really. They just travel between realities and think they've travelled through time, due to the impossibility of telling the difference between time travel and parallel reality travel, for certain.
If Spock somehow found his way back to his own reality - and how could he ever be totally sure that it was his original reality? - the timeline would be unchanged. He didn't even really change the timeline in the reality he travelled to.
Since there are an infinite number of parallel realities, everything that could possibly happen, does happen in at least one. So there is going to be one and probably a lot more realities where Nero blows up Vulcan. There's no way around it - it
has to happen in at least one reality. So even if Spock saves that Vulcan, somewhere else, another Vulcan is being blown up. So why bother saving Vulcan when it just gets blown up anyway?
By doing nothing, Spock was doing the most logical thing.
What's the point of preserving the original time-line if there are no new TV shows or movies in the works outside of the new Trek universe? It just seems demented.
Who says that some future
Star Trek movie or TV series couldn't return to the Prime Universe and just forget everything that JJ made up?
And personally, I tend to like things that are demented, but that's just me.
