Why did trade along the silk road flourish during the time of han, kushan, parthian, and roman empires

Trade flourished because these progressing empires were developing new ways to produce products and utilize agriculture in their areas, such as Han with silk and Rome with olives and fishing/seafaring. These empires has never had access to goods that were from other areas before the Silk road, causing trade to become immensely popular and relied upon. 

Apolitical machine ~ apex