Because it is not what you saw happening that counts. It's not even what you aimed the camera at that matters. It is the actual scene as it's recorded on film or videotape that has to provide the visual evidence for the audience of what occurred while you were there.
Even describing it in narration wouldn't do. The evidence simply was not in the footage.
A documentary is existential. It has to stand on its own.
All filming, and especially documentary filming, is tentative because you can't know which footage will be used, or how the footage will be edited, until you see how the finished documentary goes together.
If a film - or a video - isn't composed primarily of visual evidence, then even though you recorded it with a camera and show it on a screen, it really isn't a film.
Yes, you always hope for a great sound bite that will drive home a point. But if all you have is people telling about the topic, you lack the visual evidence to make a documentary film.
First, during shooting, it's important to keep firmly in mind the fact that the documentary is going to be edited in order to organize it to communicate with an audience.
Communicating with an audience through an existential, visual medium is far different from communication in a face-to-face or voice-to-voice situation.
When the visual evidence is well realized, however, you can get a sense of the situation in a flash.
Try not to rely on interviews to make your case.
Shoot people doing what they do, even if you're mainly interested in what they have to say.
Clearly, you have to be careful in editing, not to distort the evidence.
Every documentarian knows he's got something going if he has evidence on film or video that contradicts what the speaker says.
As a documentarian, your job is to find, record, and organize visual evidence to make a powerful, dramatic statement on the screen.