Why? Well, the U.S. won't tell us why, and the judge won't either. There are materials that the judge has seen that are "classified." As the judge said, "if accurate" this information would warrant denial of the visa.
But how can Ibrahim contest the "accuracy" of information she can't see? How can we, as citizens, evaluate the actions of "our" government if we can't see the information that the government says is the basis for its actions?
Short answer: individual justice and our collective ability to hold our government accountable will never survive if "our" government says we can't be trusted with the facts.