The Mediterranean, in part, allowed civilizations like Ancient Greece, Egypt, and Rome to flourish by providing fertile land and readily accessible trade routes. Why do we not see a similar effect along the shores of the Black and Caspian Seas?
There were scythians. These places were further away from the Fertile Crescent and they didn't favour establishment of well grounded civilization, because ever since Attila this region became known as the wild field. Basically you couldn't effectively establish a less nomadic civilization, because once you did, the effectiveness of your states military dropped significantly. Then you just get steamed rolled either by neighbours or by a new invasion force. In practice it took the eastern Slavs centuries to completely control the area, and in large part this is due to firearms, which eventually gave the advantage to agricultural economies, because they were better suited to producing these types of weapons.
Tldr. This area was dominted by the nomads, because of geography. Check out "the wild field" on Wikipedia.