Nate Silver seems to be saying the writing is on the wall at this point

[JonLP24]

70. Sanders came in first or 2nd place in most states

I won't say they choose a better candidate as I think several candidates were better than Joe Biden. You're right in the sense that that Biden outperformed Hillary Clinton even though they made the same mistakes (Iraq war, etc)

If you actually looked at Sanders record and policies in many cases he was ahead of his time and he didn't make the same mistakes Biden or Hillary Clinton did. Sexism is the only explanation I can come up with but I supported Sanders both times based on the policies. A social democratic progressive is mainstream in many countries. Basically what he advocated for is already what several European countries & New Zealand has.

As someone who followed the primary closely the odds were in Sanders favor but many candidates dropped out and specifically endorsed Biden. Then there was the media who treated Sanders early wins unlike usual primary wins especially with Carville and Chris Matthews freaking out. Also the media is a lot more friendlier to Joe Biden than Hillary Clinton.