1) The officers had no moral objections, but a good portion of the troops did. They aren't the old clone troopers, who had been conditioned to follow ANY order. They were ordinary people, with their own opinions. Supplimentary material revealed that, for example, on the first Death Star, the gunner was trying to stall for time to avoid firing. The general consensus is that also played a part in the Stormtroopers lack of accuracy against targets in non-uniform apparel (Luke, Leia, Han, etc.) 2) Support. Even planets that weren't ballsy enough to outright challenge the Empire held some degree of antipathy towards them, and would like as not harbor and supply Rebel forces as best they could without tipping the Empire off. Imagine what happened during the Revolutionary War. Even some who were against outright war would shelter troops, or provide supplies. 3) Aces. The Rebellion encouraged free-thinking and innovation, in contrast to the Empire's discipline and regimented thinking. The Rebels, thusly, had a good deal more talent that had been fostered (Wedge, Luke, and many other pilots, for example).