Scientifically speaking, when there is light, the darkness flees to the corners or anywhere that the light does not shine on. Where there is an absence of light, darkness prevails. So darkness never overcomes light.
And since certain demons are called "fallen ones", that means they fell from some former position.
Since they are "fallen angels" that means they lost the battle against the good angels. Now, that doesn't have to mean that the good angels were more powerful (although it could), maybe the good angels had numbers on their side.
Now, b/c these demons fell from their former position, they probably lost certain other powers that made them equal in strength with the good angels, but now they are not.
Anyways, this is just one idea to explain why the demons are weaker than the angels. They both could just be the same in strength.