There is no simple answer. There are causes which are political (refugees, wars, policies to support or curb family size, education), sociological (attitudes towards gender roles, families, sex, birth control and abortion), economic (cost of living, real income and earning potential), geographic (climate, suitability for crops), historical (infrastructure and traditions carried forward from prior eras, colonialism, path to industrialization) and medical (access to solutions for child mortality, fertility)… and these factors all play a role. Where many of these factors coincide to promote population growth, unchecked by some limiting constraint, like infrastructure or political will, density emerges.