The Short Answer
The Temporal Police may make no effort to save the Vulcan of Star Trek (2009) because from their perspective, it has already happened and it did not appreciably change the future in a negative fashion, or forces within the universe, restored other events that may have depended on Vulcan's existence to occur in order for that future to happen as it did. Spock Prime could likely have accounted for this for as long as he lived.
The alternative is too terrible to conceive and that is that the destruction of Vulcan caused a chain reaction through time, obliterating the Temporal Monitoring Agency and making that agency unable to return through time due to the cross-temporal travel of the Romulans and Spock Prime and are thus erased or barred from this part of the time-stream of the Star Trek universe, at least temporarily.
The Longer Answer
Monitoring the time stream is a perilous occupation. It requires you to keep track of multiple quantum realities that eventually converge on your single point of existence that you are supposed to ensure will remain intact in the event that someone from the past or the future goes back in time to alter events in the past that could lead to your future NOT existing. Woah.
The Temporal Policeman from the 31st century must contend with every era that has knowledge of time travel, accidental discoveries in the past as well as incursions from further down his own time stream where time travel is routine. What if, further in the future, the government that controls time travel is corrupted and they decide to alter the past even further.
Space is big, Time is bigger
So the job of a Temporal Policeman is finding and monitoring not the entire time-stream but the critical junctures that could alter the future in a negative or pejorative manner. Those critical junctures become the places you set up agents, or configure technology to ensure those points remain stable and unaltered. Accidents in time could change that event (i.e. City on the Edge of Forever, McCoy and Edith Keeler) or intentional incursion in the time-stream such as the Temporal Cold War in Enterprise.
It becomes perilous to alter events that change history, since you risk changing a branch that could lead to you extinguishing or cutting you off from the ability that allows you to see through time in the first place.
Not to mention, any alteration of the past probably won't destroy YOUR timeline, it will simply make the bulk of the stream of time, move in a direction different from what you intended.
Should we re-alter the now changed past?
Temporal monitoring would be difficult for anyone in general, but moving through time to alter the past without any idea of whether you would erase your present means Temporal Police will only move through time to alter events that could change the future in a catastrophic way and only with the understanding they may erase themselves from existing in the first place. So consider most circumstances from a Temporal Agency's point of view:
- Is what's happening in the past critical to my current existence?
- I still exist even after that event occurred, so should I alter it?
- Will that event significantly alter the balance of power or technology in my current temporal reality?
- If it does, how do I know? It would require me to have a record of temporal events that does not change in accordance to the time flow, a record of WHAT MUST BE for us to exist.
- Was that event supposed to happen in that fashion?
- Hard to be sure if it does not violate the first two priorities.
- Time is flexible so minor inconsistencies do not alter time significantly. Some alterations may occur due to subspace anomalies or other short term temporal events, but they are inconsequential to the overall flow of time and can be ignored. The trick is knowing which alterations have little effect on time overall.
- Think Benjamin Sisko going back in time during the early 22nd Bell riots in DS9 (Past Tense). The riots still took place but the person who was supposed to lead them dies, and Sisko takes his place. Federation records now list Sisko's face but the insurgent, Gabriel Bell's name.
- I am sure someone in the Temporal Police made a record, because the Federation made a notation from Sisko's report, noting the change and only knowing about it because Sisko tells them he changed the past! No one goes back to try to correct it because time is flexible and nothing noteworthy is changed as far as they can tell.