Republicans have had the benefit of somewhat regular wave years, while Democrats had no waves between 2008 and 2018. It has lead to a pretty substantial age gap between the "old guard" of the Democrats and the "young firebrands" with few 40-50 somethings with some experience and accomplishments under their belt that are also not knocking on death's door. Fortunately for Dems, 2020 was probably the year when that effect was greatest at the Presidential level, while 2024 and forward we will start to get more people who rose to prominent during the Trump years with some meat in their resume.
The 2020 primary situation can be understood this way:
1. Biden with a lock on ~35% of the vote with the advantages of being seen as Obama's heir and moderates believing that he would govern to the center.
2. A dogpile of youngish candidates with low name recognition who emphasized social issues over everything else. Their economic views were all over the map.
3. Bernie and Warren splitting the hardcore economic left vote while unable/unwilling to firmly position themselves to the right of #2 on any major social issues.
For a brief moment, it looked like Bernie was going to pull it off, but then he ran into a brick wall in the South because he didn't put any daylight between himself and group 2. Meanwhile, the members of group 2 who had the most potential to pull moderate votes away from Biden were scared enough of Bernie that they dropped out early. At the end of Obama's 2nd term, after Clinton's loss and the collapse of the congressional and legislative majorities, Obama and Biden were basically a 2 man army in Dem politics- the only known way to get a PV majority and an EC win in 40 years. It's only natural that the Dem base broke for the only one who was still eligible.
The most viable path to beat Biden for the nomination without a Biden own goal would have been an economic leftist candidate with a socially conservative streak. Think of the original Two-Income Trap Warren platform circa 2005. Such a candidate would have done exceptionally well in the COVID environment, too, likely having a bigger GE victory than Biden.
Going forward, and with younger candidates, the most viable outsider path is probably going to be a more polished/experienced version of Yang.
