Investigative journalism concerns itself with revealing, questioning, and analyzing a subject... rather than just reporting facts.
It is popular mainly because of its appeal to curiosity, suspense and the unique perspectives it reveals. Unlike basic bare-bones journalism, investigations look at details and suggest how those details play a role in the story. It reveals sophisticated conclusions which shape the way people think about a subject... sparks deep discussions about subjective "layers" of the story (morality, justification, long-term influence).
People love it for its sophistication... but also the fact that it's presented in a story-like manner that's easy to read... and fun to discuss.