Until 1968, presidential candidates were picked by party conventions – a process revived by Biden’s withdrawal from race

'The tradition of picking a nominee through primaries and caucuses – and not through what is called the “convention system” – is relatively recent. In 1968, after President Lyndon B. Johnson announced he would not run for reelection, his vice president, Hubert Humphrey, was able to secure the Democratic nomination despite not entering any primaries or caucuses. Humphrey won because he had the backing of party leaders like Chicago Mayor Richard Daley, and these party leaders controlled the vast majority of the delegates.'

https://theconversation.com/until-1968-presidential-candidates-were-picked-by-party-conventions-a-process-revived-by-bidens-withdrawal-from-race-235082