What is the primary reason Buck is unable to take the lead initially?

Buck's inexperience is the primary reason he is unable to take the lead initially. Throughout the early part of the story, Buck is adapting to the harsh realities of life in the Yukon wilderness, having come from a comfortable home where he had not been required to compete for his survival. His unfamiliarity with the dynamics of sled dog teams and the specific skills required to be an effective leader contribute to his hesitance in stepping up.

In contrast, the other factors mentioned—such as the strength of other dogs, fear of Spitz, or his age—are relevant but not the primary issue. Buck’s lack of experience in this specific environment means he does not yet possess the necessary confidence or understanding to take on the leadership role in a team of more seasoned sled dogs. This significant character development unfolds as Buck gains experience, which eventually leads him to become a strong and effective leader in the wilderness.