Although the idea of a "Dark Age" is mostly debunked these days, the slow unraveling of the Western Roman Empire led to a real and sustained change in material conditions. Notably, population density and urbanization both decreased, along with the labor specialization that accompanies them. I'd expect most 'inventions' to happen when and where people have the most hands on time to make them! (I can't really speak to Indian and Chinese civilizations, but they have also had integration and disintegration periods)