As a first step, let me just enumerate some hypotheses inconsistent with yours to see if they stick: (all intended to explain why higher-status people seem smarter)

• Higher status increases the amount of face you lose when you continue to believe something obviously untrue, or increases the cost of losing face.
• High-status individuals were less intelligent when they were young; the observed disparity is due solely to the wisdom that comes with age.
• High-status individuals spend more time on dinners and politics, and less time on problem-solving and reading; they exercise their minds more.
• High-status individuals are under more pressure to perform, in general.
• High-status individuals are just as smart as they ever were, but when you or I try to approach them, the status disparity makes it harder to converse with them - they would sound less intelligent if we had higher status ourselves.
• High-status individuals feel more social pressure to listen to your arguments, respond articulately to them, or change their minds when their own arguments are inadequate, which increases their apparent or real intelligence.
• High-status individuals get more honest advice from their friends, especially about their own failings (and have better friends).