I think it's for a few reasons. Many of us are interested in biographies and what made famous people the way they are. Others would like to master programming, computer science, or some other field and look for how to achieve it. Still others are trying to figure out how to quit HN without losing any of the benefits. Finally, some users fall into all of these groups. :)
Joseph Campbell is a humanitarian who researches myths, also doing quite some interpretation and trying to relate the meaning to the human life at large. It has nothing to do with software development, but (apparently) many of us find his ideas applicable, perhaps especially on the startuppy side of things, where courage etc is required.
