This and the @mattyglesias thread below it are fine. A couple of thoughts, though.

1. The system as it is tends to treat factional candidates harshly. Bernie chose to run as a factional candidate. https://twitter.com/LemieuxLGM/status/1246912911047405568

2. There is no way to know for sure who would be the better general election candidate. The bulk - not all! - of the literature as I read it suggests Bernie would have had more, and deeper, risks than Biden (or any of the others).

3. Regardless of who was in fact the better general election candidate, there's plenty of evidence that many Dem party actors believe that perceived ideological extremism is a serious general election weakness, a belief which is (correct or not) not unreasonable.

4. I don't know, but they may have also believed that a Jewish candidate, an old candidate with health issues, and a candidate with a long record of difficult stuff to defend was a real problem (obviously some of this applies to Biden too; none of the candidates were risk-free).

5. To me, Bernie's inability to shift from a factional candidate to a coalition-style one suggested real potential governing problems - that as president he might be more like the stubborn crusader of his national campaigns than like the more pragmatic pol he had been in office.

6. Obviously that last one is very much open to interpretation. But generally my own sense of how to choose among presidential candidates leans a lot more heavily on potential presidenting skills than on policy questions.