The reason is that intelligent people can be emotionally conflicted, and emotionally conflicted people necessarily make irrational decisions.
Intelligence is about drawing the right conclusions.
Rationality is about making the right decisions.
A conclusion is right if it squares with the facts.
A decision is right if it squares with one’s emotional condition.
Consequently, when someone is emotionally conflicted, his decisions are correspondingly incoherent, it being irrelevant how intelligent he is.