I believe it’s because we simply don’t understand as much as we think we do about intelligence and the brain. Partly this is a quantum problem—having to examine something that cannot be still enough to be examined—and partly it is a human ego problem. And also, as is the case with The Bell Curve, it has been a political problem.
We don’t know enough about how intelligence, generally, equates to intellectual ability in a particular direction or in regard to a particular field. That’s why someone can be a spectacular cook and a miserable mathematician or the other way around, even though both require calculation skills. Or why severely autistic people can control massive amounts of information well beyond the range of most highly intelligent people.
The Bell Curve appeared (to many) to say that white people were more intelligent than Black people. And this is not the case. But, in any event, it was a statistical account of something that cannot be statistically measured. Yet. Broadly speaking, intelligence cannot yet be measured accurately at all by anyone in any manner. Yet.