Because failure stories tend to be more sincere and heartfelt than the success stories. It provokes the readers' emotions.

Success stories usually won't include any dirty details, any shabby deals or the full naked truth of the events that underwent. No A-list actor will talk about the producer dick he sucked. You have a reputation to maintain after a success.

In a failure on the other hand, you probably are at the rock bottom, have nothing to lose, no one to be afraid of.