Because they had a lot of candidates. They figured they had to have a good one in that bunch, after all there were a lot of Democrat superstars and rising politicians. They learned otherwise. 8>)
So that the nomination of Hillary could be made at the convention in a backroom by the party’s leadership.
In this way Hillary could get the nomination without being worn out in the primary like she was 2016.
Hillary will be fresh as a daisy when she goes up against Trump this time.
Trump on the other hand (in the Democrat’s mind) will be exhausted from being President during the pandemic and impeachment.