I am not an expert in this field, but the great majority of Vitamin D is synthesized through the skin.
So, lack of sun exposure is the first suspect in any lineup. Variables in this equation include level of skin pigment, use of sunscreen, and geographical limitations. (You live where the sun don't shine much or it's too cold to be outside naked)
If you spend time avoiding skin cancer (sunscreen, clothing) you limit vitamin D synthesis.
Other things to look for would be Liver & Kidney problems, as those organs are involved with synthesizing Vitamin D into it's final form.
Also, intestinal conditions like Crohn's or Ulcerative Colitis hamper the absorption of Vit D from
food sources.
Other suspects are the usual conspiracy theories of pollution, toxins, George W. Bush, etc.
Hope this helps.
I'm going to go run around outside without my shirt on now....
-Cliff