its home prices and the ability to live on single paycheck. in nigeria you can get cheap mudhut and your wife can get a job that allows her to spend half of her time with kids. also living off of farmland instead of having industrial jobs helps you get more children, because each kid after like 4 years becomes helper on your farm, meanwhile in developed society or industrial society each kid is huge expense. this is why every society ever has stable fertility when 2/3 kids die before age of 10, then technology allows them to not die young, so population explodes, and finally fertility goes down.
Czech Republic