I was thinking the same while hoping it wasn’t the reason.
Here is a hit piece from the LA Times which has a pretty good explanation of the physics involved (April 5 2015). http://graphics.latimes.com/missile-defense/ As far as I can tell, the SBX is parked in Honolulu, resting.
According to the article: “To address this vulnerability, the U.S. had installed one land-based X-band radar in Japan in 2006, and a second was added in 2014. The two radars are well positioned to detect launches from North Korea. Yet both would lose track of U.S.-bound missiles after about 930 miles because of Earth’s curvature.”
These and maybe some ship radars and if the proper satellites are in position is probably what our guesses as to what the Norks are up to come from. Their launch directions seem to be chosen to avoid shoot downs.