Is what why?
Like other states occupied by the Russian empire, then the Russian bolsheviks (the Ukrainian bolsheviks were defeated), Ukraine underwent long periods of Russification. Under Tsarist rule, various attempts were made to eliminate Ukrainian language, culture and identity, same under the Soviets. Same reason that Moldovans speak Russian now, and many Estonians, Lithuanians and Finns spoke Russian before those countries managed to distance themselves from the Kremlin. Zelensky’s grandfather fought in the Red Army, he’s as Russified as anyone in terms of language at least.
West of Kyiv, mostly Ukrainian is spoken, in the central regions everyone is bilingual (most other Ukrainians are too), in the East, Russian is the everyday language, for the same kinds of reasons that the Scots and Irish mainly speak English rather than Gaelic.