Both questions should be asked:
Trump was elected because rural Americans were tired of liberals; nobody liked Hillary; and Putin could not imagine Trump winning, he just wanted to hurt Hillary before she becomes President.
Biden was elected because suburban nice people did not like Trump’s tweets; Biden sounded nice and non-threatening (except when bullying a fat guy in Iowa); and pandemic allowed quick-thinking Dems to increase numbers of their voters by mail vote while Trump was distracted by Covid, impeachment, illegal surveillance.