To be frank, I don't know. I just think we are in a situation where we can rule out some of the possibilities by making comparisons between earlier societies and today, as well as different countries. For example, if we assume that bad living conditions are the root cause, then we have the problem that in earlier societies with much less wealth, that has been more demanding for the average person, people tended to have more children. In addition, we see that people in quite poor countries have a lot of children. You could save the assumption by adding a hypothesis like "if people know that life could be better but cannot achieve that better life, they are less likely to have children". While this might work, we must note that inequality was even worse in earlier societies. The difference between a peasant and a member of the nobility may have been much greater than the inequality we see today (within most socienties). Maybe the peasant wasn't aware of it, or whatever.

Anyway, you need a more complex theory in this case.