I blame people's expectations, and also women's rights.
People point to poor nations, or the Great Depression, as times or places when people had/have more kids. But peoples' expectations are different there. People during the Depression didn't expect to send their kids to college, for instance; usually, they just put them to work on the farm, or in factories. These days, we don't allow child labor, and want kids to get an education for some strange reason, and that all costs money.
Women's rights and contraception are also a big issue. A century ago, women didn't have any rights. Their husbands could beat them if they got drunk and mad, and it was OK. Women couldn't have real jobs. Basically they were slaves. They could (usually) choose whom to marry, but it was a crapshoot, and divorce was usually unobtainable, plus women couldn't survive on their own very well. Plus contraception wasn't very good. These days, for some strange reason, this has changed and women now have rights, and aren't stuck being slaves to their husbands, so they have the ability to choose whether to marry at all (!), and how many kids to have, if any.
If we want people to have lots of kids again, we need to make women 2nd-class citizens without the ability to have gainful employment, we have to ban good contraception and divorce, we have to allow spousal abuse, and we have to allow children to be forced to work in dangerous and dirty factories.