1) they can't afford it, or
2) they don't have a reasonable apartment/home (same thing as 1. really)
3) they don't want to give up their lifestyle
4) they're genuinely afraid the future is dystopian and dark (which is rather interesting since I can't imagine a future more dystopian than one without children)
So if you want to fix the society to want to have children again you have to make it
1) affordable,
2) less inconvenient,
3) (not really sure what to do about being afraid of the future)
if that sounds like socialism... too bad. Capitalism evidently makes people prefer capital (both money and time) over children.