Obviously, there's one big reason: few of them could earn comparable wages anywhere else. But there's also the problem of worldview. Management's fractious relationship with the unions over decades had built up a huge reservoir of distrust among the workers, as one of my commenters points out:
I've worked at UAW assembly plants as a supplier and the average UAW worker gets their news through "the grapevine." Up until recently one of the popular views was that the big 3 weren't actually in trouble and that management was cooking the books to show a loss in order to demand concessions from the workers and break the union. It may seem silly but I have heard this from several workers at several plants. I don't know how to explain it other then most UAW workers have an absolute distrust of management.