Because the central idea of Greek life, the Idea of the Beautiful, was no longer believed by the people. Because it could not live in that form for so long. So runs idealism, and really nothing is explained. Matter exists in motion. All matter, galaxies, plants, molecules and society is in a state of motion.
Human beings are part of matter, as is consciousness, but exist in conflict with it.
That conflict is conducted through production, which discloses human kind to itself. Dialectical materialism shows that people make history not in order to unfold some concept or destiny within them, but in order to satisfy their needs. What these needs are and how they are satisfied are determined by the level of the productive forces. As the level of the productive forces changes so social relations change. Idealism sees economic relations as a function of human nature.