Easy, Saudi is not a threat to us
The hell they aren't. Most of the 9/11 hijackers were Saudi. Saudi Arabia is the source of the sect of fundamentalist Islam that drives terror, and this sect is sanctioned by the Saudi government because it's easier than trying to fight them, or to deal with terror attacks their opposition would stimulate.
They're way, way more of a threat to us than Afghanistan or Iraq ever were, but because of our oil ties and the close ties of the administration to the House of Saud we're not likely to address that threat anytime soon.