Why do you think an author would choose to use implicit main idea in a paragraph, passage, or text rather than explicit main idea?
Here are some links to interviews with popular authors discussing their approach to the writing process. Their words may give you some insight into the choices authors make as they write.
From Suzanne Collins, author of The Hunger Games:
From Stephenie Meyer, author of the Twilight series: http://movies.about.com/od/twilight/a/stephenie-meyer_2.htm
From Stephen King, author of The Shining and Carrie: http://www.npr.org/templates/story/story.php?storyId=128239303