Why were nomadic groups like the parthians able to conquer civilizations like the seleucids?

The Parthian empire was a major Iranian political and cultural power in ancient Iran and Iraq. They were able to conquer civilizations like the Seleucid as they were able to seize Media and Mesopotamia from the Seleucid making them lose control of their eastern land and it was replaced by the Romans.

d.summerains, the last one