There was, but the tradition is that an incumbent president seeking reelection rarely gets challenged during the primary. Joe Biden was the only serious candidate during the Democratic primary. When he withdrew from the race after the primary, and only a few weeks before the DNC, where the nomination would be made official, there wasn’t time to organize a second primary. It takes time to generate a list of candidates, get those to each of the states, have the states print ballots and organize a special election. The DNC was forced to pick a new candidate without going through another primary.