The Kievan Rus emerged as a major power and prospered on the back of the trade transit from the Baltic Sea to Black Sea. During the XIII century, the Black Sea trade collapsed, due to the sacking of Constantinople by Crusaders, and the subsequent weakening of the Byzantine Empire. This was exacerbated by an increasing deficit of silver in the Arab principalities farther south.
This major decay in the economic base resulted in eastern principalities (the future Muscovy) being subjugated by Turkic ethnicities that controlled the trade route from the Baltic sea toward Iran along the Volga river. The pivot to Asia included also the northern regions of Novgorod and Pskov, even though the Turkic influence there was more balanced by Swedish and German business interests.
The Kievan Rus’ heartland along Dnieper was included into the domain of Poles and Lithuanians. Before the arrival of modern agricultural techniques, they never managed to recover the economic strength of the old Rus, and remained a large, but not very significant periphery of Lithuania-Poland, and later the Polish Kingdom. Until the XVII century, Poles dominated here economically and culturally. As a result, two new ethnicities took shape: Ukrainians and Belorussians, both with their own identity and language, different from what formed in Russia farther east and north.
Simply put. Those who lived along river routes toward the Caspian Sea, became Russians. Those who lived along river routes toward the Black Sea, and further west, became Ukrainians and Belorussians.