Being outdoors isn't the same as sun exposure though. In particular, I can imagine that homeless people cover up more to prevent sunburn. Additionally, just being homeless might cause a depletion of vitamin D.
In general, I can imagine that people who spend a lot of time outdoors could get very little sun exposure since they would be more cognizant of the effects of the sun and more likely to wear sunscreen and protective clothing.
That's easy. Crappy diet and alcoholism. Something like 68% of single adult homeless credit substance abuse with their homelessness[1], with alcohol being the most commonly abused substance. Alcoholism and vitamin D deficiency has definitely been linked.[2]
In general, I can imagine that people who spend a lot of time outdoors could get very little sun exposure since they would be more cognizant of the effects of the sun and more likely to wear sunscreen and protective clothing.