Because they had no choice.

Presumptions:

• Apple didn’t want the footage to look like recorded on a phone so that viewers are positively surprised when seeing “shot on iPhone” at the very end,

• Apple didn’t want to use any 3rd-party lenses so that people think they can get the same look if they buy an iPhone (sure, not the movement because of all those drones/gimbals/dollys, but image quality).

The distinctive “shot on a phone” look comes from very fast shutter speed. The result of a fast shutter speed is that every frame is sharp (so if you pause the video, everything is sharp). The “cinematic look” can be achieved by lowering the shutter speed (so called “180 degree shutter rule”). This way you get that pleasant cinematic blur.

The problem is that when you lower your shutter speed this much on an iPhone, you get extremely overexposed image. The only solution to that is using external ND filters. Apparently Apple didn’t want to do that so they cleverly made use of Halloween to shoot at night and avoid this problem.