Aren't the Romualns right now in the midlde of political turmoil? Klingon cilvil war could happen anyttime with all those noble houses and vendettas. Let's not forget the klingon fleet takes a huge blow from Nero.
Such mishaps are not nearly enough to upset the utter domination of klingon/romulan occupation forces against mere terrorists; a phenomenon the romans (for example) - like most empires - also dealt with.
History - real world history - shows empires don't fall - aren't even moderately inconvenienced - due to terrorists.
You need a military, a large, well equipped army for such momentary weakening of the klingons/romulans to be exploitable.
As said - the notion that defeated people can overthrow their vigilant conquerors is largely fantasy.
The defeated can only hope and pray in historical happenstance - that his conqueror will be defeated by someone else, more kindly disposed toward him.
If the klingons/romulans want to keep the cardassians as as subjugated people, they can do so relatively easily. It's as simple as that.