This partially has to do with a lack of interest by historians of Iran in the Parthian empire and a difficulty in obtaining sources. Coins provide a good chronology of the reign of Parthian kings, as they do in other contexts, while more recently climate history and Silk Road chroniclers have helped to paint a more-complete picture. See Richard Bulliet’s Cotton, Climate, and Camels in Early Islamic Iran and The Camel and the Wheel for a well-argued hypothesis that Parthians as two-humped (Bactrian) camel herders helped to establish the Silk Road because of their command of these pack animals and ability to navigate and serve as a link to settled peoples and between China and Western Asia (there are figurines found of Bactrian camels carrying balls of silk from the Parthian period). But overall the Parthian empire is not credited with the cultural and wider achievements with which the Achaemenids or Sassanians (as temporal bookends) and the Kushans and Hans to the east are credited, unfortunately. It should also be noted that the stirrup and Parthian shot were hallmarks of military and horse-riding development under the Parthians.