Fishel's post from another thread came to mind...
Quote Originally Posted by John T. Fishel View Post
the theoretical position he noted was stated by Ted Robert Gurr in his 1970 classic, Why Men Rebel. It may be summarized as follows:
1. The absolutely deprived/oppressed do not rebel.
2. Neither do those who are doing well.
3. Rebellion happens after a period of social/political/economic improvement is followed by a sudden and relatively steep downturn causing the government to lose its legitimacy.
Gurr calls this "perceived relative deprivation."
It's still the best theoretical explanation for insurgency out there.
With that in mind, I would guess (and this is a pure guess - not even an "eduated" one) that most democracies tend to experience less drastic social upheavals than other forms of government. Using the theory above, less drastic upheaval means less intense insurgency (if any). The weaker insurgency gets defeated.

Another uneducated guess: in a democracy, change is less likely to take the form of armed upheaval because a democracy, in theory, gives everyone at least some voice or the perception that they have a voice and they are less likely to lash out. Hitler comes to mind. He didn't wage an insurgency. He got elected.

Third thought: an insurgency can gain legitimacy by demonizing a king or oligarchy. Who does it demonize in a democracy? The people? That doesn't sound like much of a rallying cry. "We suck - let's overthrow ourselves!"