We're not told why the man came to the Yukon, nor are we told about his traveling there. Why?

The man is in the Yukon looking for work. No detail is given because the focus of the story isn't on the man's search for a job, but rather the conflict between man and nature. Note, the reason he's there is cited in the text;

"He was bound for the old claim on the left fork of Henderson Creek, where the boys were already. They had come over across the divide from the Indian Creek country, while he had come the roundabout way to take a look at the possibilities of getting out logs in the spring from the islands in the Yukon."