I think in a lot of ways this is a continuation of the Poland-Russia rivalry to be the savior of the Slavs. This rivalry has been a major factor in Ukraine’s history, for instance the Polish-Lithuanian Commonwealth absorbed a large Western chunk of Kievian Rus during its disintegration at the same time Moscow consolidated much of the Eastern Rus principalities. It may simplistic to say this is the reason Ukraine and Belarus have separate histories but this was at least a major factor.
They have different histories because of the swamps that were only eradicated in 20th century. Try to find autmobile maps in 1930s - there are virtually no roads along the current border of Belarus/Ukraine.
