The storm troopers were ordered not to hit the rebels, because Grand Moff Tarkin hid a tracker on the Millennium Falcon and wanted to track the rebels to the rebel base. Can't track them if they're dead. There are multiple lines of dialogue in the film explaining this.
And storm troopers famously can’t hit the broad side of a barn, for that matter
The storm troopers were ordered not to hit the rebels, because Grand Moff Tarkin hid a tracker on the Millennium Falcon and wanted to track the rebels to the rebel base. Can't track them if they're dead. There are multiple lines of dialogue in the film explaining this.