When the iPhone takes a video it is, in essence, taking pictures as fast as it can. When you take a picture you have to open the camera's "shutter" (the iPhone doesn't really have a shutter, but it is a good metaphor), let in the light that represents the picture, and then close it. If the environment you are taking a video of is darker you will have to open the shutter for longer to get enough light. The fastest the iPhone can move its shutter is 15fps and the slowest it is willing to go (before it just accepts a dark or even black exposure) is 6fps.