Drivers struggle to see pedestrians at night for many reasons. Reduced visibility is perhaps the biggest; with less light, pedestrians simply blend into the shadows and drivers have a hard time spotting them. For instance, a driver turning left at a light may not see a pedestrian in the opposite crosswalk and could turn directly into them.
An issue that many people overlook, though, is known as "night myopia." To deal with the conditions, the eye attempts to focus on objects that are far closer to it. In many cases, these are too near to the driver, considering the speed of the car. At the same time, this close focus means that the eye blurs distant objects. It may still see them, but they tend to blur into the background and it's harder for the driver to make out exactly what they are -- especially when moving quickly and trying to process all of the other information that goes along with driving a car.