Gender also likely played a substantial role in Klobuchar’s failure to catch on. Biden last won an election on his own — as opposed to running as vice president — in 2008, and that was in always-blue Delaware. Sanders is a democratic socialist who has only ever run in always-blue Vermont. Neither can really make a great case for their electability at the national level. But Democratic voters consistently rated Biden and Sanders as the most electable candidates, which probably reflects their strong performance in head-to-head polls against Trump and the fact that both are white men. It’s not totally clear that any woman running in 2020 — four years after Trump defeated Hillary Clinton — could have won the nomination by running on an electability-focused message.
If you take a “lanes” view of the primary, Klobuchar was also getting squeezed out by Buttigieg. There may have been room for one Midwestern, moderate alternative to Biden, but it seems like there wasn’t room for two. Klobuchar lost that mini-primary to the former mayor. Buttigieg surged to the top of the polls in Iowa in November and although he dropped back into the pack after a while, he went on to, essentially, tie for first there, then finish ahead of Klobuchar again in the other three states that have voted so far. Why did Buttigieg outperform Klobuchar? Maybe gender was a factor here too — perhaps some voters saw Buttigieg as electable because he is a man. But I also think that Buttigieg presented his biography (veteran, small-town mayor) in a compelling way, while Klobuchar struggled to distinguish herself from the other candidates running.
Finally, Klobuchar’s campaign lacked support among black and Latino Democrats.