Thomas Schelling hazards an explanation:
Assume the sale [of fissile material] succeeds. The terrorist organization needs the people who can convert the fissile material into an explosive. It needs several highly trained scientists in physics, chemistry, computer science, and metallurgy, and highly skilled machinists and others who can produce something technologically demanding. The fact that a bomb design can be found on the internet, doesn’t make it easy. Anyone can find out how to make a Chevrolet, or an MRI or a CAT scan; there’s no secret, but it’s not easy!