Systems select for power, not necessarily intelligence. Power involves organizing and building loyalty. Often, the power seeker benefits if all around him are weak and stupid. What loyalties you gain are not strictly through lines of common intelligence but common affiliations.
I mean, it's a good question. Why hasn't human civilization naturally developed into a brutal aristocracy of the highly moral and high-IQ? Why don't we have a global "130 IQ high-trust Anglo" paradise already?
What mysterious force obviously overcomes and beats intelligence?