IMHO one major reason is "Lack of Meritocracy"

Brain drain has interesting (but of course not exact) parallels to voluntary migration: here too, there are push and pull factors, particularized by the country that is experiencing brain drain and the country that benefits from it, but also by individual factors. On the push side, the main causes of brain drain are political instability, unemployment, or limited career opportunities but economic underdevelopment, unsatisfactory working conditions, the poor state of research facilities, discrimination, and lack of freedom, singly or in unison, can also be explanatory variables. On the pull side, higher paying jobs and a better quality of life are the main attractions but others can be a more stable political environment, a more promising economic outlook, the prestige of higher spheres of academia, intellectual freedom, etc. One can expect that skilled people in fields such as science, technology, and engineering would, perhaps more than others, experience such push and pull factors.

'Brain drain' happens when highly skilled professionals are not satisfied with material conditions in their home country. In my view one of the causes of brain drain is 'excessive materialism' and the belief that one will get ahead in life only by amassing wealth; that prosperity and 'development' are merely material in nature and that social relationships (with parents, friends, relatives etc.) can be easily traded off vs. money and material comforts.

Why science, technology, and engineering? Because those are the fields in which there is demand, ease of access, and mobility, unlike the situation for people with softer skills or for professionals in certain fields (e.g. medicine) in which their credentials aren't accepted at face value and they must go through longish retraining or recertification periods. The folks I know who left from Pakistan and the Middle East were more concerned about stability, security, freedom, and a better life for their children than excessive materialism; they lead modest lifestyles and their desires for comfort and a material safety net are quite middling.