Why didn't Dr. Strange know about what's happening to the Infinity stones, or other realms?

Question

Dr Strange told Thor that he is monitoring all 9 realms in Thor: Ragnarok. So how come he didn't know what's happening at/to Nidavellir, Titan, or the Infinity Stones.

Strange: My name is Doctor Stephen Strange, and I have some questions for you. Take a seat. Tea?

Thor: I don't drink tea.

Strange: What do you drink?

Thor: Not tea.

Strange: So, I keep a watch list of individuals and beings from other realms.

Answer 1

Doctor Strange says that he monitors individuals and beings from other realms (planets such as Asgard or Knowhere, dimensions such as the Dark Dimension, etc), not the realms themselves. So long as the individual is not the ruler of their realm/planet, he can presumably leave their own law enforcement to monitor them when they are nowhere near Earth.

As an equivalent: the CIA or MI6 may monitor specific foreign nationals who have been flagged as potential threats, but they do not monitor everything going on in every country in the world - for one thing, they don't have the resources/manpower! Doctor Strange is thus highly unlikely to be able to monitor more locations with less resources.

Also - Doctor Strange takes over from the Ancient One after the events of "Captain America: Civil War". This means that, for example, during the events of "Avengers" or "Age of Ultron" he was still either in training, or had yet to travel to Kamar-Taj, and hardly in a position to intervene.

Answer 2

How come he didn't know what's happening at/to Nidavellir, Titan, Infinity Stones.

Because he's not all-knowing.

He can maintain a watchlist of known players but if he doesn't know about them, they won't be on the watchlist.

It's absolutely clear that Dr Strange does not know about Thanos since when Banner says "Thanos is coming."...Strange's reply is

Who?

There's no indication that Strange has ever heard of Nidavellir or Titan and at the stage we join the action, Thanos has only just acquired a single stone so it's arguable he has no reason to be on Strange's mystical "radar".

Answer 3

Strange is watching over the Earth. He is not actively monitoring the beings / realm.

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He never said he is monitoring all 9 realms. His exact lines were

So, I keep a watch list of individuals and beings from other realms that may be a threat to this world. Your adopted brother, Loki, is one of these beings.

He did not care what Loki was doing to Asgard, not even sure if he was monitoring Asgard. He only intervened after Loki came to Earth.

Also I think that Loki was included in the list not because he was a Villan but because he tried to attack and conquer Earth. Thanos's affiliation to the attack is not known to anyone before Banner told them. Since Thanos has not attacked Earth (according to them at that point) he might not be on the list or he might not have known Thanos at all.

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So how come he didn't know what's happening at/to Nidavellir, Titan, or the Infinity Stones.

1. Nidavellir was a peaceful planet. It was no direct threat to Earth. It is possible that no one on Strange's list was on that planet and so he was not monitoring it.
2. Titan was assumed as a dead planet with no life. Why would anyone monitor it?
3. The Infinity Stones were in many hands before and the only stones that affects the people of Earth were the ones on the Earth. The Power stone was with Nova core, Ether will the collector, Mind stone with Vision, Time stone with Strange and Space stone with the Asgardians. The infinty stone were with people who will not use it to destroy life / attack Earth. So Strange will be less concerned about it than the beings such as Dormamu.