“One of the things we love about the still image is the way in which it can stimulate the imagination to create a fiction around an image. The fact that we can commit a single image to memory in a way that we cannot with video is a big reason photography is still used so much today.”
In short, photos are more memorable. They also tell a piece of the story which leaves the rest open to the imagination. Videos reveal too much, removing the mystery of the still moment.