Some event of mass death anyway. It's sort of like how in rainforests the only place with enough light for new trees to grow is in places where old trees have died.
Thinking about it that might just have been caused by Alpha.
Remember when Saturno was told "You were bred to be smart?"
My guess is, a bunch of new spawn were purposely allowed to live, introducing fresh, blissfully ignorant demons to the world of life. Alpha, in need of a new omega, watches the entrance[s] to the human world in case any spawns, smarter than the others, can learn anything about the human world. It just so happens that one of them did.
When Saturno exited his... pod thing, he wasn't met with a big ole puddle of spawn. He was met with dozens of different, older creatures all attacking all at once. A scrap of food was introduced to his environment, yes, but it wasn't from a huge destructive rampage that wiped out everything.
It's interesting to think about how a ecosystem that only has one species in it would function. Sure, the life stages are so different from each other that it seems to function similar to most ecosystems, but still...
It makes sense that when big things die, all the spawns are able to live more than a few seconds.
That last panel is... kind of sobering though.
Intense!
