Its because they usually have different goals:
fine art photographers usually care about aesthetically pleasing but "useless" photographs where photojournalists care about capturing the news rarely really caring about aesthetics. Unfortunately its not the fault of the photographers themselves, but of the market.
Fine Art collectors usually don't want something tied to an event, unless it has some historic value. New agencies care only about how well a photo would illustrate an event. Very few photographers care about combining both, for example the Magnum Photo agency and photographers such as Sebastiao Salgado. There is a reason the agency is by invite-only and Salgado is often critised for "beautifying" misery and ugly events.
My belief is that a good photographer will make good photographs even in a war zone or where speed matters, sometimes even standing by themselves as great pieces of art. Most of classic photography is actually documentary, but where aesthetics and art takes a mere historical event even further.
As a journalist, I am often in a hurry to find photos to accompany the stories I write and its not unusual to prefer a "boring" snapshot that illustrates the text better than a good artistic one.

But its a large and good subject and would like to read more opinions.