Why do people use mandolins over food processors
Precision, mainly. I use one for small amounts when I'm fussing about uniformity. My KA processor, like most others, is great at many things, but precision slicing isn't among them. Seems to be a baked-in problem. Besides, I'm sometimes too lazy to fuss with the set-up and clean-up chores a processor involves.
I use a mandolin for fine slicing of different thicknesses because a food processor cannot give me the control that I need. I rarely use my food processor for savory dishes because it is so much easier to accomplish the task with a sharp knife when I'm cooking for 2-3 people.
A mandolin is also much faster to clean up than a food processor.