Perhaps because there are banally more people ... A larger number of people born with an equal percentage of mutations will give a proportionally larger number of those born with a deviation. In India, with 1 billion inhabitants, there are simply more people with deviations from the norm than with the same probability of occurrence of these deviations in a country where, for example, there are only 100kk inhabitants. Plus, of course, poor ecology due to the growing volumes of industrial production (due to the huge amount of labor that needs to be occupied) and neglect of the sanitary and hygienic state of life and human settlements in general.
In India, a disgusting environment and a considerable number of people are employed in extremely hazardous industries that poison the body and spoil health. Accordingly, the chances of giving birth to a completely healthy person in such people are reduced.
In Vietnam, many children with physical anomalies were born after the war - although the war itself ended, American reagents, such as Agent Orange, remained in the soil, retaining their influence on human genetics.