Why did many former colonies often experience difficulties when they achieved independence?

Colonies established under New Imperialism were primarily seen by Europeans as sources for the Raw materials needed for Industrial production, and as markets for the finished goods. As such, European powers did not create much industrial infrastructure. Further, while Europeans did establish schools and hospitals, they were primarily staffed by Europeans, and most of the colonized individuals were not given oppurtunities in higher education. Thus when colonies became independent and all the Europeans went home, they left behind underdeveloped infrastructure, and populations that did not have many educated officials (due to European policies, no one should ever blame the colonized for these problems).