Well, to an extent it has: there's archeological evidence that human intelligence has risen from ~40,000 AD to ~10,000 (see "Before The Dawn"). It's just that additional intelligence (beyond what we have now) doesn't seem to confer any further evolutionary advantage.
It could be as simple as an energy tradeoff. Our brains already consume a significant fraction of our daily calories, having them work harder might not improve our reproduction results sufficiently to balance the increased risk of starvation.