Because the term "unprocessed" carries its own ambiguities. Is cooking a steak a form of processing? I would say so. What about the mashing of potatoes? Ultimately, processed and clean are largely subjective.
http://www.sciencebasedmedicine.org/index.php/should-you-be-...
People just call clean, whatever they like/think is beneficial. Everything they deem harmful is unclean.