Vitamin D deficiency, when serum 25-hydroxyvitamin D is lower than 25 nmol/L, occurs in risk groups all over the world, mainly in the Middle East, China, Mongolia, and India. Risk groups for poor vitamin D status are children especially those with low birth weight, adolescents, pregnant women, older persons, and non-Western immigrants. A vitamin D status can be considered adequate (serum 25-hydroxyvitamin D >50 nmol/L) in less than 50% of the world population at least in winter.
Worldwide Vitamin D Status

Part of it is lack of sun, some of it is cultural (see the Middle East). I think a some of it has to do with the amount of time that people spend at work.

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