The Transparency International policy paper on Poverty, Aid and Corruption highlights the correlation between the amount of foreign aid available in a country and the level of corruption experienced by that country. This is largely as a result of two problems: the people who administer the aid are not accountable to the recipients, and aid projects are typically not subjected to rigorous evaluation.
The intended aid recipients never get to hear about how much funding they were supposed to have received, and they would have no-one to complain to if they were to complain about the misappropriation of their funds. As a consequence, aid money is easy money for corrupt politicians and administrators.
Western donor governments want to give aid to the poorest countries. The problem is that the poorest countries usually have corrupt governments. Accordingly, the donor governments look for ways to justify giving money to bad governments. They give money to particular countries as a reward for being ‘slightly less corrupt’ this year than they were last year, or as a reward for political policies such as supporting the ‘War on Terror’.