Doctor Strange didn't trap Thanos in a loop using the Time Stone in Dormammu by trapping him in a neverending time loop, dying over and over until the demon agreed to his bargain and left Earth alone.
Just two years later, Doctor Strange found himself fighting another near unstoppable force who threatened mass destruction: Thanos. While he used all manner of magical spells on the Mad Titan, he seemingly never considered trying the same Dormammu time loop trick again. ittedly, there is a lot of debate over what exactly Doctor Strange was doing in Infinity War - after seeing 14,000,605 futures, he decided to give Thanos the Time Stone, enabling him to ultimately kill half the universe - but this one aspect seems a cut-and-dry oversight. Even Thanos himself questions it during their fight.
Doctor Strange Putting Thanos In A Time Loop Wouldn't Have Worked
There's actually a lot more reasons why Strange didn't put Thanos in a time loop than it at first seems. The most obvious comes from those 14 million futures Doctor Strange saw; it's entirely possible he did attempt to use the Time Stone in one or more of them, only for it to not succeed in ultimately stopping Thanos.
Indeed, Dormammu was from the Dark Dimension, an alternate world that is removed from any conventional understanding of time. Allowing him to sense the age, even within a loop, was key to wearing down his defenses (and rounding out Doctor Strange's exploration of the fear of death). As a being of our dimension, Thanos may not be quite as susceptible to a similar trick, especially given how driven he's presented as being. His victory in Avengers: Infinity War ultimately comes down to his sheer force of will and ability to do what the Avengers and Guardians wouldn't to achieve his goal. Being trapped for millennia would certainly have an impact, but as presented, Thanos is a true unstoppable force.
And a time loop isn't quite an immovable object. The other key difference between the two situations is knowledge of the Time Stone. Dormammu isn't just new to the concept of time, he doesn't understand the sorcery Strange is employing (who didn't know it was an Infinity Stone). In Infinity War, Thanos' primary goal on Titan is to get the Time Stone from Strange, bringing it out of wherever he'd hidden it to create a loop risked exposing it to attack.
While none of these is a resolute explanation, they show that Doctor Strange putting Thanos in a time loop may not be as foolproof as it may initially seem. After all, to truly beat him required first having him win. As seen in Earth's Mightiest Heroes' ultimate victory over Thanos in Avengers: Endgame, it proves Doctor Strange correct in believing the timeline in which he surrendered the Time Stone to Thanos was one in which they'd win. Unfortunately, it meant the loss of both Iron Man and Black Widow, but it's likely that Doctor Strange putting Thanos in a time loop in Avengers: Infinity War would've resulted in even more casualties because it wouldn't have worked.
