Why is bilateral vitiligo bilateral?
Er… the too short answer is that mammalian embryonic development is bilateral. If the issue is genetic, it is likely affecting (or affected by) the chemical push-pull of intercellular communication during development. We develop bilaterally, thus bilateral vitiligo. Contact dermatitis and things that one catches environmentally localize around the point of contact. Thats why it’s extremely unusual (but not impossible) to catch bilateral poison ivy.