They did choose Biden, (and his v.p.)
Biden resigned.
There is not time for a new primary under the State election rules. It would be ridiculous and unfair to any new candidates to expect them to be able to campaign for just 2 to 3 weeks, or come up with the campaign funds to run a decent campaign against their opponent in the presidential race.
Not enough time to settle divisions within the Party, with several candidates hopping in 3 weeks before the convention which is unfair to new contenders...no new candidates are hopping in to the race under this time limitation.
And under Party nomination rules that govern this situation at the convention, with a candidate dropping out, it frees up the delegates who represent their state voters, to vote for a new candidate, (which could be after the 1st round of voting?)
Divided we fall, united we stand...
This Hail Mary, only has a chance, and a daring chance at that, is to unite around one candidate.
The voters are not complaining that voted for Biden in the primary that Biden chose to drop out of the race.