Is it true as described by Zecharia Sitchen that the Anunnaki seemed to have abandoned direct interaction with humans some time just before Alexander the Great, and if so, what led to this decision to go into the background and abandon direct physical interaction and authority over humans?

This is largely correct, and was due to the fact that their involvement here in orchestrating a visible hierarchy, much like a dynasty of Kings, was on the wane because of the change in position of their home planet necessitating many to leave, as they will only maintain a small outpost here, for the most part, when their planet is too far away in distance. It is part of a regular cycle they engage with. And so there is a coming and going of influence and direct manipulation. It is because they are engaged elsewhere, not only on the home planet, but other outposts throughout the galaxy. So they have many activities and many hobby-like involvements with varying civilizations, and in their lust for power and thirst for new conquest, frequently hop from place to place to cause mischief, and when they begin to tire of a project will give it less and less attention. That is what has been happening in these intervening years of less visible presence in being in the forefront. They have maintained their grip, but more indirectly, through their Anunnaki bloodline inter-mixed with human through breeding. This they have left in place to mind things and they have less direct control and manipulation. They are present in the physical still, and you personally have seen an Anunnaki, as we have told you before. And so that is evidence they are still out and about, and those who can pass for a physical human will have a varied role in carrying on missions of surveillance to have the direct experience of interacting with humans and human events. This, again, is done as much out of curiosity as necessity, but allows them opportunities to intermingle safely.