Because they show us a world different from the garish one we see around us, they draw us into their mysteries.
The consistency isn’t only to do with style. Black and white strips an image to its essentials, and I think this lets the emotions come out more powerfully. The little looks, the fierce grip of a child’s hand to their parent, their brother or sister, their favourite bear. I believe we look at black and white images more deeply. Because they show us a world different from the garish one we see around us, they draw us into their mysteries. They suggest rather than showing or shouting.