Why do some fruit flies have white eyes?

Due to genetic mutation. Flies' genes can mutate into a form that prevents any pigment from getting into the eyes. When a fly receives two genes called "recessive genes" from it's parents that cause the fly from putting color into it's eyes then they appear white. If they only have one gene that tells them to have white eyes then the more "dominant gene" will prevail and cause them to have another color eye such as red.