To make money.. There's no money to be made in Sri Lanka... There is in BD.. GDP per capita means nothing in this sense.. It's about how much money is flowing through BD and how many bucks are up for grabs.. same reason English people come to Arabia to teach English..
Actually we have qualified locals.. it is just that the system is stacked against them and they leave abroad.. when they are at home, nobody hires them as everyone thinks a foreigner will always do a better job than a local.. The whole issue is borne out of sheer misconception of industry leaders.