Without being too scientific, I would hazard a guess at saying that contributing to open source can come down to good old fashioned economics, human interests/motivation, and overall social culture.
Maybe this can best be qualified by an open source contributor from a Scandinavian country - but maybe life in the Scandinavian countries allow people to be more focussed on pursuing their personal interests, assuming most of the base necessities of life are taken care of in their environments, such as being able to access quality education, or being able to earn salaries that can take care of most of your needs in life without, having to work overtime like a dog.
According to the latest Zeitgeist movie, societies that are more equal, in terms of not having wide social stratifications as compared to others, tend to have a situation going where the citizens in those environments usually contribute more towards their overall social development freely. Plus there're other things, like less crimes, political problems/corruption, etc. as a result.
Elizabeth provided an answer voted as the best answer in this link that seems to paint a picture of life in Scandinavian countries where people don't like working long hours, prefer working jobs to make money for free time, prefer to spend time with their families, are not inclined to be wanting to be wealthy, where there is no wide wage gap differences based on the type of job you're in, and where people seem to be more interested in your interests and hobbies rather than what you do for a living. Seems in line with idea put forth about social equality and life problems caused by the monetary system according to the latest Zeitgeist movie.http://answers.yahoo.com/question/index?qid=20100313142831AAyJ9FS
So according to Maslow's Hierarachy of Needs Pyramid, it may be that most open source developers are functioning in environments that allow them to be closer to the top of the pyramid as opposed to the non open source contributors who may be functioning in environments that put them closer to the bottom of the pyramid: http://en.wikipedia.org/wiki/Maslow's_hierarchy_of_needs
Think about what you'll be spending most of your creative time doing if all your physical life sustaining needs in life could be met without you having to work at least forty hours per week.