Because he had to win an election. The opposition knew that he was opposed to vaccinations and had planned to use this against him in the campaign. Can you imagine how he would have been attacked for being an anti-science anti-vaxxer in the middle of a pandemic, albeit a fake one? So he championed the vaccine, but dropped many subtle hints along the way that something else was up. In due time, we will out find out what really went on behind the scenes.