As we know in the MCU, Doctor Strange becomes the Sorcerer Supreme after battling Dormammu when the Ancient One gets fatally wounded. He takes on the mantle as Earth’s protector, and the same thing happens in What If…?. However, in What If…?, this Doctor Strange is still grieving the loss of Christine, whereas, in the main MCU timeline, he isn’t grieving at all.

Because he has control over time using the Eye of Agamotto and the Time Stone, he tries to bring Christine back to life by going back in time. However, as Doctor Strange continues to fail and grow frustrated in bringing Christine back, the Ancient One appears to tell him that it isn’t possible.

Her death is what leads him to seek out mysticism, eventually become the Sorcerer Supreme, and gain power over time to bring him back to the moment of her death.