It's all the malnutrition in India.
Nigeria and Pakistan rank 2nd and 3rd in the world in stunted children with 10.3 million and 9.8 million stunted children in each country.
Now comes the part that everyone will downvote me for.
It's because we are overpopulated.
Whenever I'm in India I'm blown away by the height differences between Indians from state to state.
Certain states, due to their massive populations, have such widespread, long-term, multi-generational, malnutrition problems, that they are much shorter than their counterparts in other states.
FTA
The average guy in Punjab has a height of 168.6 cm which is around 5'6.4" and an average Jharkhandi lad has a height of 162.7 cm which is around 5'4".
It's because Punjabi food is so rich in calories, also Punjab has never had a population problem, and there's no history of widespread starvation or famines in Punjab.