Mainly, Buck needed to be brave and strong, but he also needed to be be clever and to learn about how a dog team worked.
When Buck got to the Yukon and began working as part of a dog team, he quickly became strong. He had always been brave. But those were not enough to make him qualified to lead the pack.
Instead, he needed to learn the ways of dog sledding. He had to learn what the men needed from all of the dogs. But more importantly, he had to learn how to be clever. It was only by being clever that he was able to beat Spitz. He used his wits to undermine Spitz's authority so the other dogs would not respect him any longer. That really helped Buck be able to take over as leader.