I don't mean to be a "dreamkiller" because I want Greece added to ABD as well. But from what I heard, because ABD tends to lean towards families going on these trips together (parents, children, grandparents), usually the time of year that everyone can go together is summer when the kids are out of school. Greece is very hot during the summer and I think ABD feels the grandparents and the kids can't handle touring in the heat.
Not sure how much of that is accurate, but I would love to be wrong and see Greece added to the list.