There are a number of explanations:
The demons aren't organized, and since it's every demon for itself, they actually get in each other's way.
Demons don't actually have an easy way to get to the world.
Primal spirits/gods/etc keep the world safe for their own reasons. Even if the gods all have different alignments, they might all be willing to work together to keep other creatures from stepping into their territory, each for their own reasons.
Demons have something inherent in them that prevents it (i.e. it requires some kind of energy to manifest in the world and so they can't remain for long, or they are only capable of manifesting when certain conditions are met).
Demons prefer the Abyss. This is similar to the previous reason, but in reverse. Maybe the Abyss is a great power source for them. Maybe they relish the challenge of a real fight, and the world doesn't normally provide it (i.e. the world is a fun little playground for awhile, but they'd rather go up against the big boys to show off their power). Maybe the world is somewhat uncomfortable to them, or the Abyss just feels like home.
There is some kind of cosmic balance that prevents it. I'm not a fan of this one, as I prefer explanations wherein the PCs can make a difference in the grand scheme of things, and this seems to limit that.
The demons aren't organized, and since it's every demon for itself, they actually get in each other's way.
Demons don't actually have an easy way to get to the world.
Primal spirits/gods/etc keep the world safe for their own reasons. Even if the gods all have different alignments, they might all be willing to work together to keep other creatures from stepping into their territory, each for their own reasons.
Demons have something inherent in them that prevents it (i.e. it requires some kind of energy to manifest in the world and so they can't remain for long, or they are only capable of manifesting when certain conditions are met).
Demons prefer the Abyss. This is similar to the previous reason, but in reverse. Maybe the Abyss is a great power source for them. Maybe they relish the challenge of a real fight, and the world doesn't normally provide it (i.e. the world is a fun little playground for awhile, but they'd rather go up against the big boys to show off their power). Maybe the world is somewhat uncomfortable to them, or the Abyss just feels like home.
There is some kind of cosmic balance that prevents it. I'm not a fan of this one, as I prefer explanations wherein the PCs can make a difference in the grand scheme of things, and this seems to limit that.