Quality. The problem tends to come down to human trials. Very few countries have a combination of the quality of medicine, the proper oversight, and a large enough population to actually make the trials worthwhile. China/India simply don't have the same quality control as that of US/Europe/Japan.
Beyond that, humans aren't all equal. Drug trials in the US make an effort to represent the general US demographics within their studies (which is also representative of Europe). Clinical trials of a 100% Indian population have questionable relevance any other country, and will likely require the entire study to be redone with a population-representative sample.