Because the Democratic Party establishment shoved Mondale down the throat of the Democratic Party voters. Not entirely dissimilar from 2016. Same ultimate result. It didn't matter in 1984; no Democrat would have stood a chance. But the Democratic Party establishment forgot history, so they were condemned to repeat it in 2016.
I voted for Gary Hart in the primary. So did my older brother. This was even after our labor-union father handed both of us a written list of the Mondale delegates to vote for.
Democratic Party voters voted for Hillary by like a 3 million vote margin over Sanders... and she's highly popular among dems. Not sure where the whole "shoving down Democratic voters throats" come from. Nice #HotTake.