It is considered difficult to create culture-fair intelligence tests because they:

I believe the answer is: reflect the cultures of some test takers more than others

culture-fair intelligence is a form of Iq test that is designed in a way that minimized cultural bias.

For example, language could be considered as a bias with a test because children that grow up in a neighborhood for example, would feel harder to grasp the test language that is commonly used for students in upper middle class.