It’s because the election system is First Past the Post. If they changed it to Ranked Choice Voting, all candidates from both primaries (plus independents) could run in the election and it would be up to the public to decide which of them is best, with their vote going to a fallback candidate if their favourite doesn’t win.
Very well argued. This is quite unrelated but just out of interest, how realistic do you see constitutional reform in the US? Because to me as an outsider, that seems to me to be what US politics sorely needs rn. Is there appetite for it among the electorate? Among the Dems? Could it be conceivably carried out?
That would make sense