That, or perhaps a lack of sun exposure is linked to having preexisting conditions. Diabetics, COPD, asthmatics, etc as a group probably tend to spend less time outside.
There's nothing particular that is required in the diet to synthesize Vitamin D3 from the sun.
Without sun exposure, you'd have to be eating several pounds of fatty fish every day to get to "healthy" levels from diet alone. You need a supplement.
Keep in mind, non-severe Vitamin D deficiency doesn't cause any significant evolutionary pressure, so migrating humans just rolled with it - to this day.