Three national/ethnic groups trace their ancestry to Kievan Rus': (Great) Russians, Ukrainians, and Belorussians. The latter two diverged from Russians mostly due to the long period of Polish-Lithuanian rule, which brought Catholic and Latinate influences quite distinct from Orthodox Muscovy. But why did Ukrainian and Belorussian split from each other? Both were under Commonwealth rule for about the same amount of time; both later came under the rule of the Russian empire and then the USSR. As far as I can tell, the two countries never were part of separate polities until 1991.

Edit: this thread asked a similar question. I feel that the split from Russian is explained well there, but there's not much on why Ukrainian and Belorussian are separate.