Steam sterilization is a time proven and economical process of killing microorganisms through the application of moist heat (saturated steam) under pressure. Heat damages the cells essential structures including the cytoplasmic membrane so that the cell is no longer viable.
The rate by which bacterial cells are thermally inactivated depends on the temperature and the time of heat to which they are exposed. In practical terms this means that it would take a longer time at lower temperatures to sterilize a population than at a high temperature. Additionally, the higher the concentration of organisms that need to be killed, the longer it will take to kill all of the cells in that population at the same temperature. In principle this mechanism is valid also for irradiation and EtO.
Furthermore steam cannot be used for goods with tight packing. It is much more suitable in a hospital setting where all packaging and wrapping can be penetrated by steam.