How about this kind of intervention? Is that intervention enough?
Ukraine's former president Viktor Yushchenko says he hopes Europe will wake up to the threat posed by Russia in the wake of the poisonings in Salisbury.
Fourteen years ago Mr Yushchenko was taking on a presidential candidate favoured by Russia when he was poisoned with a dioxin, a toxic chemical.
Or this, re. 2019:
While the interference did not live up to worst fears, numerous examples of it can be found in the kinetic, disinformation, and cyber realms over a period of months. Russia’s war with Ukraine and its occupation of parts of Ukraine’s territory constitute the most blatant interference, including the disenfranchisement of some 16 percent of the electorate living in Crimea and areas around Donetsk and Luhansk.
Heightened vigilance by Ukrainian authorities and civil society helped to reduce its potential impact. In contrast to 2014, when Russian cyberattacks compromised the Central Election Commission network, Ukrainian authorities were more prepared for possible attacks in 2019. As a result, during the first and second rounds of the presidential election—despite numerous minor cyber incidents—Ukraine did not suffer a major cyberattack.
As the question asks:
why Russia couldn't or wouldn't want to prevent Zelenskyy from taking office?
Also seems that many have short memories. Until recently many were claiming Zelensky was too accommodating to Russia, so why would Russia have gone out of its way to avoid his winning?
Or, even more amusingly, pre-election, Why a Zelenskyy presidency would be a disaster for Ukraine:
Fourth, Putin and his minions in the breakaway regions of Ukraine’s southeast will be delighted with Ukraine’s progressive decay under Zelenskyy’s mismanagement. Who better to lead the country they consider a joke than a clown? Who better to fail at coping with a possible provocation in the occupied territories, the Sea of Azov, or mainland Ukraine? Putin will be sorely tempted to launch an armed attack on, say, Mariupol just after Zelenskyy’s inauguration. Perhaps even more dangerous would be a charm offensive that seeks to entrap the naïve Zelenskyy in a set of obligations that amount to Ukraine’s abandonment of its sovereignty.
Furthermore, a pre-election INFRI study, Kremlin-linked forces in 2019 elections states that Russia was primarily interested in getting rid of Poroshenko. Which did happen. Then, according to the study, Russia, having ensured no one too anti-Russia got elected, would just re-engage into negotiations with whichever individual won.
Last, Russian interference in the US, is often assumed to seek to promote one particular candidate, while in fact many experts are of the opinion that merely poisoning the electoral debate and pushing Americans against each other is closer to their actual intent.
One suspects too, that by 2019, being Russia's preferred candidate would have been somewhat of the kiss of death in Ukraine, making the whole premise of this question - that Russia could push "their" guy to victory, somewhat unrealistic.