Why Bernie Sanders keeps failing?
…and could he have succeeded?

Socialist ideas favor a consumer point of view and are less considerate of the provider or producer point of view. Consumer point of view is easier to comprehend. Someone who has taken a service or used a product could have a consumer point of view. Consumers are numerous. Providers are few. In order to gain electoral majority, politicians in democracies worldwide embrace socialist platforms to campaign.
Successful socialist politicians campaign for one significant change or limited changes
Problem with broad changes
Bernie Sanders advocates revolutionary changes in almost all areas of American lives— universal healthcare, free college education, higher minimum wages, guaranteed minimum income, … (did I miss some?)
Broad changes inflate the pool of electors who can empathize with a provider point of view. Plus, a provider will likely be significantly more sensitive to the risk to their livelihood, compared to a consumer’s moderate desire (or tolerance) for change.
Consider Universal Healthcare. As a patient who have experienced problems with US Healthcare system, I would welcome change.
However, if I am a talented doctor or nurse at a popular, well run clinic, the new healthcare system would demote me to just another minion in a complex price regulated world. I will need to queue up behind my less hard-working peers, with a ton of paperwork, to secure a lower, fixed, government regulated compensation, for working on terms set by the government. An outcome that I would dread.
If I am a healthcare researcher exploring cure to a rare disease, I may lose my job when price regulation kills the economics of my project and ends medical research in the field.
A moderately paid private university employee may appreciate universal health care and may initially be inclined to support Bernie, but with the proposal to offer free college, would dread the government set quality and regulatory mandates that would come with free tuitions. Such a voter would rally against Bernie, joining the doctor, nurse and the medical researcher.
A barely profitable restaurant owner who hasn’t been able to send his daughter to college or consistently pay for quality insurance may initially be excited for change. But not so much when he realizes that the minimum wage changes will likely warrant firing some of his decade long employees to maintain prices. Worse, his business may not survive another disruption. The uncertainty of losing the livelihood could have made this person a vocal opponent to Bernie
End Result — Bernie’s supporter base disproportionately comprises of millennials, who are likely too young and inexperienced to strongly empathize with any of the provider point of views
A divide and conquer approach, focusing on a single issue would likely have delivered better results for Bernie, and possibly an electoral victory. But his strategy advocating broad change likely ensured that less people supported Bernie in 2020 than did during his first campaign against Hillary in 2016, as more of his initial supporters empathized with various provider perspectives.
Image Courtesy: Gage Skidmore (CCA-SA2.0, Wikipedia Commons)