This has been studied by sociologists and it basically comes down to people having more opportunities outside of the home to make money. For poor families, kids are viewed as an asset since you can use their labor to earn income through farming or other services. For higher-income homes, having a kid means you might have to reduce hours at work, miss promotion opportunities and so on. So people decide to not have kids.
It's harder to compare housing but fertility rates are declining even in countries with free/cheap healthcare, daycare, and university.