Power

Power doesn't necessarily corrupt but it tends to select for corruption.

As an evolutionary characteristic, power enhanced breeding success. Social customs developed from animal behaviors before the emergence of conscious thought. After the development of language, the power of bullshit could be added to physical qualities such as size and combat skill. Along with thought and reason comes misdirected thought and faulty reasoning. Along with the naturally-occurring errors these can generate, they provide leverage for manipulating people through mistaken perceptions.

Fair-mindedness would cause an ethical leader to step down over certain issues in which a corrupt leader would look for some way to excuse remaining in power. Because various margins for error coincide nicely to provide these justifications, the traditions of leadership tend to accumulate greater tolerance for corruption up to a certain point. Beyond that the dirt becomes obvious. This might not cause a leader to topple if the leader can command enough forces to stay on top of the heap, but it commits that leader to the role of despot rather than merely "flawed" or "controversial."

Uncorrupted leaders still manage to operate even now. Because they tend to limit themselves through their ethics they are still in the minority, where they can be expected to remain. The system as we accept it still favors people who grab for power and cling to it tenaciously. We even praise the qualities of energy and ambition as signs of the ever-valued work ethic almost regardless of the outcome.

Where a would-be leader of old might need to command fighting forces to bolster that ambition, now the seeker of power needs to command money or solicit donations of it. While this can be less destructive in the obvious sense of riding roughshod over the countryside pillaging, it raises corruption to new importance as the supplicant makes deals in return for financial support.