The problem for GM was that when the sales slowed down, they had trouble cutting costs because most of their costs were fixed. In other words, a lot of their costs did not go down as their sales went down. In most manufacturing companies, when sales go down, some of the bigger costs go down as well (if you aren't selling as much of your product, then you cut back on manufacturing through layoffs, through reductions in material purchases, and so on).
GM has tremendous fixed costs related to their union contract. Closing a plant, for example, did not necessarily mean the workers lost their jobs.
Company pensions and legacy health care costs were fixed as well. So when sales went down, many costs stayed fairly constant. And that led to losses. --
Why GM Failed