There are as many answers as you have imagination: they’re all undead, they’re all infertile due to magical radiation, they sacrifice all their children to Moloch, they eat babies, a monster comes and steals them all, etc.
Ultimately, it comes down to that Asshai is a Thin Place more than almost anywhere else in the world:
…beyond the walls of Asshai little grows save ghost grass, whose glassy, glowing stalks are inedible. If not for the food brought in from across the sea, the Asshai’i would have starved.
The ships bring casks of freshwater too. The waters of the Ash glisten black beneath the noonday sun and glimmer with a pale green phosphorescence by night, and such fish as swim in the river are blind and twisted, so deformed and hideous to look upon that only fools and shadowbinders will eat of their flesh.
Every land beneath the sun has need of fruits and grains and vegetables, so one might ask why any mariner would sail to the ends of the earth when he might more easily sell his cargo to markets closer to home. The answer is gold.
Beyond the walls of Asshai, food is scarce, but gold and gems are common…though some will say that the gold of the Shadow Lands is as unhealthy in its own way as the fruits that grow there…
The dark city by the Shadow is a city steeped in sorcery. Warlocks, wizards, alchemists, moonsingers, red priests, black alchemists, necromancers, aeromancers, pyromancers, bloodmages, torturers, inquisitors, poisoners, godswives, night-walkers, shapechangers, worshippers of the Black Goat and the Pale Child and the Lion of Night, all find welcome in Asshai-by-the-Shadow, where nothing is forbidden. Here they are free to practice their spells without restraint or censure, conduct their obscene rites, and fornicate with demons if that is their desire.”
This is a place where “nothing is forbidden everything is permitted.”
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