Why did Buck refuse to rise and lead the team? What did he have that his masters lacked?

I assume you are talking about what happens towards the
end of Chapter 5.

There seem to be a couple of reasons why
Buck does this.  First, he has lost the will to live.  The men have treated him and the
team so badly on this trip that the "spark of life" is about to go
out.

Second, the book says that he has a sense of
"impending doom."  I would say that this is what Buck has that the men do not.  He
senses somehow that the ice is not going to hold them.