why wouldn't chris stay in carthage when westerberg asked him to stay om for a few more weeks

From the text:

By mid-April, Westerberg was both shorthanded and very busy, so he asked McCandless to postpone his departure and work a week or two longer. McCandless wouldn’t even consider it. “Once Alex made up his mind about something, there was no changing it,” Westerberg laments. “I even offered to buy him a plane ticket to Fairbanks, which would have let him work an extra ten days and still get to Alaska by the end of April, but he said, ‘No, I want to hitch north. Flying would be cheating. It would wreck the whole trip.’”