I can only answer for myself, but because Biden had a better chance of Winning, and because a radical like Sanders would likely get very little done. I’m personally much closer aligned to Sanders‘s ideology, I just don’t think for national Office He is positioned in a place that he could accomplish much.
What you say above is the big lie CNN has told Democrats and why CNN is more dangerous to Democrats than FOx.
It is now undeniable that Bernie style populist policies almost across the board are what the vast majority of Americans on the left and right want. The Independents and middle class and working poor and below all want real help to rebalance the economy back to where it was just a few generations prior before this massive wealth shift up to the top richest Americans.
Polling shows that the reason voters have given up on Democrats is because while they campaign on being Populist they deliver very little and often nothing.
That is because CNN says a 'Proper', 'Centrist', 'Moderate' Democrat is Joe Manchin types and people like Bernie are labelled "Extreme', 'Socialist'. CNN absolutists represents the Corporate/Donor class and will smear any attempt to get populist policies in place as extreme.
Joe Manchin and not Bernie Sanders would have been labelled the extreme in the Dem party in any fair analysis. Voters overwhelmingly want populist policy and Manchin was against all of it, while pushing a corporatist agenda.
Steve Bannon and Marjorie Greene both recognize this and both are positioning themselves to run on a full Populist agenda, hoping to unite right and left populists, which would be the most dominant voting block in American history. It is something Democrats should have and could have done a long time ago, but CNN would attack them for every time they even leaned that way trying to scare Americans to not vote for them.