During his time tormenting The Avengers, Thanos was easily the most destructive character in the Marvel Cinematic Universe. With his formidable skills and a rampaging army at his side, the Mad Titan attacked many a planet, not to mention the countless he murdered in his quest for the Infinity Stones. So, with all that going for him, it's a bit curious that he never actually attacked Asgard.
After the events of 2012's The Avengers, Thanos had to have known that the Space Stone, aka the Tesseract, was going to be in Asgard following Loki's arrest. But even with the location of the gem, he never went after it or any of the other stones until much later. The reason for this remains a mystery, but it's possible that Odin's presence may have been a major detractor.
Odin May Have Searched for the Infinity Stones Before Thanos
Odin had the means and powers to actually fight Thanos. In fact, he might have even previously attempted to attain all the stones himself during his time colonizing the nine realms, as alluded to by Hela in Thor: Ragnarok. The Allfather was so infatuated with the idea of that power that he even had a replica made of the Infinity Gauntlet.
Obviously, given the contents of his throne room, Odin knew a lot about the Infinity Stones and the power they possessed. With that being the case, it's likely that he also knew of Thanos and any others who were seeking that same power. Unfortunately, Odin, not being the most open person in the world, kept a lot of things close to his chest, especially when it came to his children. Even after Loki worked for the Mad Titan, the Allfather didn't fill him in on the possible dangers lurking in the stars.
Thanos Feared an Attack From Odin and He Might Have Been Planning One
However, it's possible that Odin kept his kids in the dark because he was preparing to take on Thanos all along. Given Asgard's extensive army and defense system, it stands to reason that the King of the Nine Realms would've had a contingency plan for a run-in with the Mad Titan. Thanos commanded the Chitauri army, and while Rocket described them as the "suckiest army in the galaxy" in Avengers: Endgame, they still posed a threat to any realm.
As for the reason Odin didn't strike Thanos first, his last few years were plagued by tragedy. After an attack by the Dark Elves led to the murder of his wife, Frigga, he was placed under Loki's spell and sent to Earth, where he would spend his final days. It was during this period that Thanos began actively searching for the Stones, meaning it's possible that Odin would've made a preemptive attack; he just didn't get the opportunity.
Now, all of this information about Odin could be convincing enough for one to understand why Thanos never attacked, but the real smoking gun came in Avengers: Infinity War. After the destruction of Asgard, the realm's refugees boarded a ship and set out for Earth, only to be attacked by Thanos' forces, meaning he practically waited for their home to fall to strike. Without Odin, the Asgardians didn't stand a chance, and half of their population was decimated during his attack. It remains just a theory for now, but without knowing it, Loki may have given Thanos his easiest victory to date when he sent his father packing to that retirement home.