I just want to echo what KRKappel said about the resources required to invade a system like Mon Cal (I imagine the same can be echoed for every other planet in one degree or another).
Physical resources are one thing, intelligence is another. But beyond fleets there would need to be build up of stores, the logistics of the actual assault. All of this would be a drain on the resources of the empire in some way, be it from established front line units or reserve forces. There is examples of times when the cost/benefit analysis to deal with a planet or area of space ended in no action by the empire, or at the least very limited action.
The biggest example I can think of is Hutt Space as an area left alone. A contrary example of a planet that required a massive amount of forces to secure and that draining anything from its units to project power elsewhere would of been a disastrous choice is Corellia. That planet alone had over 20 star destroyers around it.
Basically, asymmetrical warfare on a galactic scale is a complicated beast and even the empire would have to pick its battles.