Here's why:
* Passive voice emphasizes the action being done (losing jobs) rather than the doer of the action.
* Active voice would focus on the subject (the depression) and its action: "The depression caused skilled and unskilled laborers to lose their jobs."
In the original sentence, the subject is "skilled and unskilled laborers" and the action is "lost jobs." The sentence doesn't explicitly state *who* or *what* caused them to lose jobs, which is a characteristic of the passive voice.