I think you're drawing conclusions from coincidence. If I had to pick a reason for the disparity, it's that public transit and dense city centers make less sens

I think you're drawing conclusions from coincidence. If I had to pick a reason for the disparity, it's that public transit and dense city centers make less sense in countries with large, rural areas. Rural areas tend to have lower levels of education, and level of education has been linked with birth rate (specifically, higher female educational achievement leads to lower birth rates). The three areas you list as "developed nations with well developed public transportation" are all compact and densely populated. The three you listed as "less developed public transit" are much larger, with a significantly higher rural population. I would bet that if you normalized for education, you would see any disparities between your cohorts disappear.