I think it's because most of life is melancholy. Heck even the funny/happy songs I've written have some pathos in them. Just look at how many songs are structured, stable root, build tension, then resolve. The middle is longer than the ends kind of like when you get out of your warm comfy bed, go through your tension filled day and then resolve to your comfy warm bed at the end of the day.

A leader at a songwriters conference said that the happy/funny songs just don't have the staying power even if they do become hits.

Marc, I remember you once made a comment about how the sad songs written would fill a football stadium while the happy songs would fill a swimming pool.