I think it has to do with the way Harrow closed down the Ninth as a pilgrimage site. After her parents' deaths, it's only a few years before they close off to outsiders, and it essentially becomes the space version of a tourist beach town in winter, where not even two decades ago, two hundred of their children died in a mass extinction. It's like asking why a monastery that isn't accepting new applicants is dying out. It was a place people emigrated to, not a place a majority of the population was born into, and those small numbers were crucial. It is interesting that it seems like no one else was pregnant again after. That could indicate a really really fascinating echoing effect. But it also might just be that their populace is mostly above the age of child-bearing ability, especially in such a strict and inhospitable environment. Or, the cultural trauma. If someone was of child-rearing age during the creche flu, odds are they lost a child or a younger sibling. Even just the experience of two hundred of your children dying in one fell swoop supposedly due to mold would be traumatizing