Maybe “Mediocracy” is to blame. From a recent NBER Working Paper:
In this paper, we study the initial recruitment of individuals in the political sector. We propose an equilibrium model of political recruitment by a party who faces competition for political talent from the lobbying sector. We show that a political party may deliberately choose to recruit only mediocre politicians, in spite of the fact that it could afford to recruit better individuals who would like to become politicians. We argue that this finding may contribute to explain the observation that in many countries the political class is mostly composed of mediocre people.