Hard to tell exactly why, because it was not depicted. During heavy storms, most people would be on call, attending watches in the engine room and bridges. It is implied something more sinister happened, i.e., that the crew members may not have abandoned ship as they didn't have enough time or, if they did, they died while escaping. Most likely, they died during the storm (several causes, as such heavy trashing, being swept away, the freezing cold etc.), but their bodies were not shown, as it was irrelevant to the plot.