I don't think this is all that accurate. Not only is relative cultural uniformity in Eastern Europe a relatively recent phenomenon - Slavic political and by extension cultural expansion to the Volga Region, for example, only started in the 16th century - but the East European Plain isn't as open as it might appear, either. Before the Early Modern Era, this was a vast, sparsely populated forest with some of the largest swamps in Europe.
Not to mention that Eastern Europe is massive. Just the region between the Vistula and the Urals is larger than the rest of Europe combined. There is nothing preordained about a single culture group dominating that whole area.
Once a strong, centralized state rises in the region (which is bound to happen due to the conductive geography in the area; vast tracts of flat land, major rivers running through), it also leads to some level of cultural and ethnic unity. The original post's premise is if Kievan Rus' could have evolved into ethnically distinct states, which I doubt. He's not asking about if Eastern Europe in the 10th century was diverse or not.