well to refresh Admiral Thrawn was the essence of Loyalty. Otherwise he wouldn't have become a Grand Admiral and then try and rebuild the Empire. Also there isn't much money in the Rebellion, those that defected did for other reasons.
The second Death Star had the protection of the shield generator so why move when you have protection and the drive systems probably weren't working. And remember that it was the site of the Imperial ambush they didn't want the death star to movie or else there goes the ambush.
And the Empire was responsible but they still would have a hard time to win:
1. The rebels would run when they could not get a space superiority, when they did they would destroy a couple of targets and leave before the empirer shows up.
2. The empire has to protect certain locations and can't leave them unprotected. The rebelion didn't have any thing that they had to protect, they could move with freedom. Indicitors came later on so......
3. The cell structure of the Alliance, prevents the empire from finding out where the alliance HQ was at.
4. The rebels had nothing to lose and everything to gain so they fought harder, the empire had everything to lose and little to gain and took soldiers off planets they conquered so those men wouldn't care as much about the empire as those that joined volentarily.
It was the same way in Vietnam.
"Dulce bellum inexpertis."
(Sweet is war to those who have never experinced it.) Roman Proverb