1. Because wealth depends on your social network. Humans are a social species, the more social you are, the better status you have. That's how business works. Intelligence is often the inverse of being social.

2. Even if one is more intelligent, he/she is not that much more intelligent. Of course intelligence matters, especially in certain fields, but overall intelligence will not allow people to gain an edge, or at least not an edge that will make a big enough difference.

3. Wealth and status stems rather from certain moral values, rather than abilities. Of course modern society tends to value new ideas and innovation, but human societies, in general, still favor a certain set of traditional values that make it harder for intelligent people to get favors. And in general, innovation breeds change, and change always brings its share of problems. You can look for anti-intellectualism.

Not to mention that intelligent people don't really seek wealth at all, they know it's much preferable to seek recognition from their peers.