It's not that highly intelligent people make poor politicians. It's that people who are regarded as "highly intelligent" are often just people who are hyper-focused in one small area of life (being an "intellectual") and have neglected other areas that are needed to be a well-balanced leader.
Developing social skills to a high degree of competence involves a large investment of time and thought. It's just not something that academia awards PhDs in
I never thought he was bright...