He was able to ascend to the level of Sorcerer Supreme is a very short amount of time (at least in the MCU) despite others who have been training for decades or centuries.
Is there some explanation for his apparent uniqueness?
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I'll split this into two parts. The first part will be to talk about Doctor Stephen Strange himself, and the second part will be a comparison between him and other sorcerors (e.g. Wong, Mordo)
There are a few motivating factors for Doctor Strange that are somewhat unique to him:
Most of that makes him extraordinary, but not unique. In a similar situation, you could expect someone like Christine Palmer, Howard Stark, Hank Pym**, etc., to all be at a level to match that kind of ability. Many of the Sorcerors also probably similar. So what distinguishes Strange from a sorceror like Wong, or Mordo?
Aside from Kaecilius, no one else knows that the Ancient One is siphoning energy from the Dark Dimension. Even Mordo is surprised by this.
*debatable initially, since the warnings came after the spells.
**I specifically sought some of the non-enhanced intellectual geniuses in the MCU for that comparison. Bruce Banner or Loki could arguably belong there, but one is sorta transcending humanity, and the other is a God, so ...
Just after Doctor Strange used Eye of Agamotto for the first time:
Wong: You weren't manipulating the space-time continuum, you were wrecking it. We do not tamper with natural law. We defend it.
Mordo: How did you learn to do that? Where did you learn the litany of spells required to even understand it?
Doctor Strange: I've got a photographic memory. It's how I got my M.D. and Ph.D. at the same time.
Mordo: What you just did takes more than a good memory. You were born for the mystic arts.
These conflicting statements are the best canon can explain. Basically, neither Mordo nor Doctor Strange himself knew why he was special.
This question is similar to "Why Hank Pym or Howard Stark was smart?": Doctor Strange was enthusiastic about learning new things. He didn't use to played with rulebook, so he never hesitated to experiment. With his long history in die-hard academics and research, his brain was highly flexible to adapt to new learning and he could concentrate well with good creativity. All these aren't true for other sorcerers.
Also, Doctor Strange didn't become Sorcerer Supreme within few days. For example, he had initially hard time using sling ring and fighting with Kaecilius. Ancient One also clearly looked more capable. It took several years for Doctor Strange to become Sorcerer Supreme, so he also showed high resilience and grit.