Petro Poroshenko was delaying the transition of power because, in exchange, he was bargaining with Washington for security guarantees. Poroshenko wants to be sure that after stepping down as President, he will not be imprisoned by the Ukrainian oligarch, Ihor Kolomoisky and his people. Such guarantees were imitated by Washington through the opening of a criminal case against Kolomoisky. This would demonstrate to Poroshenko that in order to avoid extradition, Kolomoisky would not dare to leave Israel and return to Ukraine.
However, after the vote on Volodymyr Zelensky’s inauguration passed in the Ukrainian parliament, the Americans broke their guarantees. This is seen by Kolomoisky’s immediate arrival in Kyiv just hours after the parliamentary decision.
Generally, Poroshenko lost the presidential elections because his external curators disliked two things. First, Donald Trump did not forgive Poroshenko for the arrest of Trump’s lawyer Paul Manafort on a tip off from Poroshenko. The second reason was Poroshenko’s autonomous ‘foray’ in the Kerch Strait.
It was indeed reported in early April 2019 that the FBI is investigating potential financial crimes by Ihor Kolomoisky. However, there is no proof that the investigation began because the US authorities decided to imitate certain security guarantees to Petro Poroshenko which were later violated, as alleged in the article.
Robert Mueller's 448-page report, which mentions Ukraine a dozen times, mostly in a part which assesses Paul Manafort's ties to Russia and Ukraine, does not contain a single mention of Petro Poroshenko.
See earlier cases alleging that the US introduced anti-Russian sanctions in exchange for anti-Trump damaging information from Poroshenko’s team and that the US did not allow Petro Poroshenko to win as he began leading the US into a war with Russia.