Few things.
1. Summerset seceded and Valenwood was lost due to a political coup
2. Morrowind and Black Marsh seceded after the septim dynasty fell
3. The lands that the Dominion asked for in Hammerfell were already conquered by the Dominion, in a war of conquest that's usually what happens doesn't mean that the Empire views the other provinces as expandable.
4. Hammerfell had no part in the sacking of orsinium, it was Redguard mercenaries, not hammerfell itself.
5. The Legion actually saved and helped a lot of Orcs out of Orsinium but due to how High Rock's political structure is they couldn't risk arresting all parties involved without causing more political problems which is a bad thing since High Rock's political structure is more cutthroat then anything in Tamriel.
6. Cyrodiil was in no strength to continue the war and rebuilding against an enemy that takes longer to rebuild is a better strategy then going all out into a war again especially when morale among troops was probably extremely low and a lot of citizens and soldiers were either injured or killed.
anyway moving on from "empire is filled with corrupted politicians that only care about imperials"