Most answers I see are rationalizations for the status quo, but really, I don't think there is a good reason for this state of things. Most programmers seem to believe not having a life is part of the deal and spoil it for the rest of us. Managers are good at screwing everybody around, unions are a joke, everybody says the economy sucks, but then there's always a need for programmers, and this should be exploited by the category.
Also I'm kindakind of flustered no-one pointed out that a wonderful reason for asking for a part-time job, would be "having a kid."
As soon as a critical mass of people starts asking for part-time programming jobs, there will be plenty of offerings. So go on, and convince more people to do the same. This is the closest we get to a union battle.