Mistaken belief that he won the ideological battle
Bernie Sanders repeatedly claimed that "our movement has won the ideological struggle". Sanders insisted that a majority of the American people agreed with his signature policy proposals, including his plan for Medicare-for-all and free college for all, regardless of income.
However, there is no evidence to supporter Sanders' contentions. A December 2019 New York Times found that Joe Biden's proposals were more popular with Democrats than the proposals of Bernie Sanders. For example:
58.4% of Democrats agreed with Biden that the U.S. should offer government-run insurance to anyone who wants it, but people should be able to keep their private insurance if they prefer it
24.7% of Democrats agreed with Sanders that the U.S. should adopt a national health care plan in which all Americans get their insurance from a single government plan
34.7% of Democrats agreed with Biden that the government should take steps to make college more affordable, but most families should still have to pay something
30.5% of Democrats agreed with Sanders that the government should make public colleges free for all Americans, regardless of income. See complete poll results in chart below.
Notice that the other health care and college proposals that some preferred were more moderate therefore closer to the Biden's proposals than Sanders' more progressive ideas. Also, these are the opinions of Democrats and Democratic leaners. It is fair to assume that most Republicans and Republican leaners would tend to favor less progressive legislation.
If the results of polling are not enough to dispell the notion that Sanders won "the ideological battle", the primary elections results should be. Joe Biden won 56.9% of the Democratic primary vote to the 43.1% that Bernie Sanders won. With that sort of popular vote gap the argument from Sanders' die hard supporters that the media or the establishment somehow caused Bernie's losses strains credulity. It is much more likely that Sanders proposals simply were less popular than the alternative proposals.
Misplaced faith in young voters
Bernie Sander based his hope for being elected president on the misplaced faith that he could inspire and motivate young voters to vote for him in unprecedented numbers. That didn't happen. See the chart below.
How Huge Voter Turnout Eluded Bernie Sanders on Super Tuesday
(New York Times, March 7, 2020)
In state after state, there has been little evidence — at least outside California — that he has generated higher turnout among young voters. And though he has promised to deliver record turnout, it may in fact be Mr. Biden who is accomplishing that, lifted by his strong support among black voters.
In Texas, where Mr. Biden prevailed over Mr. Sanders, only 15 percent of voters were younger than 30, and nearly two-thirds were 45 or older, according to exit polls. The age breakdown was similar in California.
In no state did people younger than 30 account for more than 20 percent of the electorate, based on exit polls, and in most states they accounted for 15 percent or less.
Because so few young people voted, it did not matter that Mr. Sanders won them by huge margins, because Mr. Biden won the much more plentiful older voters.
Mistaken belief that voters want extreme consistency from their elected officials
Voters rate open-mindedmess as a highly preferred trait for their elected officials. In other words, they don't like its opposite, ideological rigidness. Voters like politicians, who like themselves, have views that evolve and change over time.
At a CNN town hall Sanders said that all prisoners, including domestic terrorists such as the Boston Marathon bomber, Dzhokhar Tsarnaev, should have the right to vote while they are incarcerated. Another more adroit politician might have found a middle ground on the prisoners' rights issues, such as returning voting rights to released prisoners. There is must be small portion of the electorate that is concerned with Dzhokhar Tsarnaev's voting rights. A larger portion is likely to be repulsed by any policy that favors granting any new rights to Tsarnaev.
Heading into the Florida primary, Bernie Sanders in a "60 Minutes" interview stood by his past praise for certain policies of Fidel Castro's government. Democrats across Florida, including prominent members of Congress, erupted in dismay. Maintaining his mantle of authenticity which is a beloved characteristic by his supporters, Sanders stubbornly refused to temper his apparent admiration for the late Cuban dictator.
The result in the Florida primary was Joe Biden winning 62% of the vote to Sanders' 22.8%, Bloomberg's 8.4% and Buttigieg's 2.3%.
Mistaken belief that voters don't care about labels
Anybody who understands packaging knows that a product sells better when customers are familiar with the brand and the label. Would-be presidents and other candidates have been wearing the Democratic Party label for 192 years. It's an well-known and familiar brand name with solid customer loyalty.
Bernie Sanders (I-Vt.), though he has run twice for the Democratic presidential nomination, has steadfastly refused to actually join the Democratic Party and to drop his self-ascribed label of "democratic socialist" or "independent". Many Democrats probably harbor some resentment and suspicion, perhaps unconsciously, for any politician who won't join the fold.