Historically its because at one time both factories of Canon & Nikon were in the same location close to the Yukkimoto mountain in Japan, millions of years ago a large part of this mountain fell into the sea and left a huge cliff- face behind. As the space at ground level wasn't great one of the factories had to be built right up to this cliff-face, and Nikon chose to site there as it was mostly in the shade from the Sun - hence of the overall site they were on the 'Dark side' and the name kinda stuck
Dave