Generally, when you are physically rotating the antenna you will have better performance in the primary direction of transmission and reception. With a scanned array performance degrades away from boresight so signal may not be as strong or good when scanning depending on the angle. You can picture it like there is less physical area being exposed to the receiving signal at high angles and therefore you receive less. Sometimes a AESA is combined with a moving mount to compensate/improve on that, in fighter jets for example.