When the switch connect AC, the current is zero 120 times per second ( for 60Hz US systems ) or 100 times/sec ( for 50Hz ). When the switch is turned to "off", the current continues to arc across the contacts. But as the AC voltage is zero those many tmes a second, the arc is extinguished very quickly. However, DC voltage is never zero, so any arc across the contacts would remain forever, or until the switch burns up. Thus, the DC rating must be much lower to prevent perpetual arcing.