All of Rosver’s points are valid reasons.
Plus, there are more planes than Astlan and many don’t have as advanced of defenses.
And not all fourths can transport that many people.
But also…the demons are rather parasitic. They need goods and services and the Abyss doesn’t have a lot of natural resources. So most demons who can transport stuff physically (vs fade in fade out) prefer to make money and accumulate power by being traders/importers.
Further, most of the demons capable of such travel have been around for quite some time, or are pretty smart to begin with.
Sure, mass slaughter is fun for a few centuries, but ultimately boring and not very fulfilling. Particularly since humans die a lot and aren’t much of a challenge.
BUT….and this is critical for BOOK 2….and it really trumps all other reasons….all the previous are just excuses used by archdemons and princes.
The reality is this:
There are Gods.
The Gods depend on their worshipers on the Planes of Men to maintain their rather exorbitant lifestyles.
So the Gods protect their worshipers.
if demons start raiding the gods’ mana generators, [b][i]they will come down and start kicking ass.[/i][/b]
And between the gods and their avatars…well…that’s not a fight a rational demon would want…