Cloud conditions, if any existed at this time, are not seen across the eastern third of the u. s. and canada in the image because

The answer is the sun has already set leaving darkness. During the time the satellite image was taken, the sunset was taking place across the eastern third of the U.S. and Canada. Cloud conditions were seen from western regions of the U.S., Canada, Texas, and Gulf of Mexico because the sun has not yet set in these areas.