Their theory seems to suggest that the Horrors that stepped from their astral home into the physical world made up a majority or at least a large portion of the entire Horror populace. I venture to imagine that not every Horror even made the journey to the barrier between the worlds.If we accept Jaron's hypothesis as accurate, we can conclude that the Horrors come from a magic-rich netherworld whose landscape thankfully remains unimaginable and inaccessible to Name-givers. The question then immediately arises: why do the Horrors leave their home plane to wreak havoc in our world? If we presuppose that the Horrors have all the magical energies and anything else they can possible want in their netherworld, then this question can have no answer. However, all too many Horrors appear eager to bridge the gap between the astral and physical realms to feast upon our land. Any Barsaivian who glances out his window at the still-scarred landscape, or listens to family stories about our forebears' time in the kaers knows the truth of this. Therefore, we can only conclude that the Horrors want or need something our world has to offer.
ED1 - Horrors pg7
I would argue that the native Horror realm contains the necessary resources for them to thrive, but within that world there is competition. The strongest of them have an easier time obtaining those resources. The weakening of the barrier between worlds, allows the most opportunistic among Horrors to breach the gap and make their way into the physical world. Their opportunistic nature is what has allowed them to survive in their world alongside superior Horrors. This world allows them access to those resources with less competition... for now.According to Jaron, the Horrors' native lands lie somewhere in the deepest (or perhaps furthest) reaches of the astral plane. ED1 - Horrors pg6
My argument is based on the comparison of Horrors to my knowledge of real-life, which is probably a mistake.
Conclusion: I know we usually play in a world with a stagnant but relatively low level of magic, meaning that the majority of the more powerful Horrors (of the ones that made the transition) find it impossible to remain. The purpose of expressing my theory was simply to suggest that the Horrors that ravaged the world during the scourge, which we think of as fearsome and powerful, are actually the weaklings of the netherworld.
I am expecting to find out that my theory was either covered or squelched by other source material I have yet to thoroughly read. Just fun to think about.