Rights issues probably. Unlike the other US Federal military and paramilitary organizations NASA's images aren't in the public domain for most uses, and they're very particular about employees being used in commercial material.
From their media usage guidelines:
NASA Content Used for Commercial Purposes
For more information on using NASA content for commercial purposes, please read NASA Advertising Guidelines. Any questions regarding use of NASA content, or any NASA image or emblem should be directed to Bert Ulrich of the Multimedia Division of NASA's Office of Communications at NASA Headquarters in Washington.
For information on NASA involvement in documentaries and films, please see documentary and fictional film project guidelines.
If the NASA material is to be used for commercial purposes, including advertisements, it must not explicitly or implicitly convey NASA's endorsement of commercial goods or services.
If a NASA image includes an identifiable person, using the image for commercial purposes may infringe that person's right of privacy or publicity, and permission should be obtained from the person.
Current NASA employees, including astronauts, may not appear in commercial material.
Commercials and promotional content cannot be filmed on NASA property
If I'm not mistaken, Dr. Porco is currently an employee on the Solar System exploration committee so unless they go find her CalTech faculty official photo, which may also be copyright, its not something they can use.