I believe the in-universe explanation (i.e. EU) is that there is a lag time between when the DS fires and when it has to charge up for a second shot. So it couldn't just fire two shots in a row; if it blew up the Yavin gas giant, the Rebels would have ample time to flee before it could charge up to fire on the moon base. Obviously, one of the upgrades the DSII had was the ability to fire in quicker succession.