I quite agree with Dana. The patriarchy lives! That said, it's not why "auto workers are unionized, but house cleaners and hair dressers aren't." The variance in union density between those professions has much more to do with the legal and political era in which the auto companies were unionized, and the differences in organizational structure between hair salon employees (diffuse, lots of small businesses) and manufacturing employees (concentrated, large amounts of machinery which create value outside of the worker). Over the last few decades, various laws have made organizing tougher, and beyond that, it's simply harder to unionize service sector employees -- particularly immigrant service sector employees -- and when you do, there are fewer gains to distribute, as each individual worker's labor creates comparatively little value.
So salon workers aren't unionized for much the reason that most service sector employees aren't unionized, and for much the reason that only 7 percent of the private sector workforce is unionized. Things just aren't going that well for the unions. Which is a shame. Particularly for salon workers, who, as The Nation usefully explains, really need some regulatory help.
Update: Folks are right, trucking was a poor example. Unions are finding it very tough to organize contract truckers, but UPS, etc, also count as trucking.
