What is a possible reason for a vitamin d deficiency?

Lack of sunlight: Most commonly lack of uv light exposure to the skin during the seasons of the year where sunlight is less. Other reasons would be gastrointestinal conditions where it is not absorbed in the diet.

Sunlight: We have been so educated that sun is not good for skin, that people use ultraviolet skin protecting lotions and creams for protection against skin cancer. However in phoenix where sun shines all the time, there is an epidemic of vit d deficiency because of avoiding the sun too much. Should have 15 minutes exposure to sunlight each day. Vit d supplements are available for treatment of deficiency.