In Avengers: Endgame, Doctor Strange along with the other Masters of the Mystic Arts were able to teleport thousands of heroes onto the battlefield during the film's climax. With that said, it's clear now that he was capable of also bringing the Titan team back to Earth in Avengers: Infinity War, so why didn't he do the same when Thanos came after the Mind Stone in Wakanda?
Well, a new fan theory on Reddit from user u/Lucaswebb proposes a two-fold explanation for Strange's lack of action during one of the most important battles in the Marvel Cinematic Universe.
THE ONE OUTCOME
The first part of the theory suggests that when Strange saw only 14,000,605 outcomes, the only scenario in which the Avengers would have defeated Thanos was exactly how it unfolded in Endgame. Strange was not able to teleport anyone to the battlefield in Wakanda as doing so would disrupt the one timeline where they were victorious. Although he was not able to see every outcome (in which there could have been more cases where they come out on top), it was just too big of a risk for Strange to gamble upon, as one wrong move could forever doom their universe.
The Titan team was also not in very good shape. The Guardians of the Galaxy were knocked out and Spider-Man was most likely exhausted from his efforts to save them, leaving an angry Nebula and an injured Tony with a severely damaged Iron Man suit. If Strange did bring the team to Wakanda, they would not have made much of an effect at all, as Thanos plows through the Avengers in a matter of minutes before getting his hands on the stone in Vision's head. Sending a badly injured Tony Stark back into battle so soon (even if he upgrades his armor via satellite the moment he arrives) is still a surefire way to get him easily killed, completely closing off the one victorious outcome.
THE DEAL
The second part of the theory suggests that by not following Thanos into Wakanda, Doctor Strange was in fact honoring his half of the deal to save Tony Stark's life, promising "no tricks." Tony needed to survive in order for the team to bring everyone back and eventually defeat the Mad Titan. As brutal as he is, Thanos is a man of his word, and if Strange broke his deal, he would have likely killed Tony without hesitation, which would ruin the Avengers' chances of winning.
There is a glaring flaw with the second argument, however, as having the team return to Wakanda would not necessarily be considered a "trick" but rather a continuation of their battle. As well, what Strange ends up doing in the film was an actual ruse, as giving away the Time Stone was the only way for them to "trick" Thanos into his own demise.
It would seem that the first part of the theory seems to be correct, as that point is made very clear in Strange's actions during Infinity War and Endgame. Every move that he has made since landing on Titan and viewing the timelines was calculated to ensure that the actions of the Avengers will lead them to the one outcome where they restore the universe and defeat Thanos. Strange could not take the risk of disrupting the one timeline where they succeed, and as a result, the Titan team is left out of the battle in Wakanda. As well, the Titan team was not in peak condition, and especially with Tony being badly injured, one wrong mistake could have completely ruined their chances of saving the universe. With all that in mind, perhaps it wasn't such a bad idea to not teleport to Wakanda.