People don’t reject evolution because they are intellectually incapable of understanding it. To phrase this as a conflict between intelligence and stupidity is way off the mark. It is fundamentally a conflict between two basic beliefs: (1) mind and intelligence can evolve from non-mind and non-intelligence; and (2) mind and intelligence must spring from a greater mind and greater intelligence. Position #2, the anti-evolutionist position, insists that the cause cannot be inferior to the effect.
Many people reject it because it's stupid. Monkeys accidentally transforming into humans. Banana-picking monkeys gradually perfecting banana-picking skills by monkey trial-and-error, thereby changing into full-blown cathedral-building humans. It's stupid.