Digging a bit deeper into the popularity of Bernie, one finds the source of his economic policy that boldly claimed we "can" afford the things other countries take for granted. That was/is his chief economic advisor, Dr. Stephanie Kelton, a longtime advocate for MMT. He knew better than to outright adopt such a radical concept that went against all of the "common knowledge" accepted by the public mid-primary and presented taxing the wealthy to answer all funding questions, but his fundamental perspective on federal finance was largely guided by her insight.
The fear of this perspective gaining traction was far more a threat to the Democratic leadership than simply running a self-claimed socialist as its candidate. The major frustration Hillary expressed when reviewing her primary struggle was that no matter what she offered in policy, Bernie doubled down on it. She, out of frustration (and her natural neoliberal tendencies) could only counter with the same "How will you pay for it?" challenge utilized by Republicans. This threatened the power base of the elite 1% by making them irrelevant to the well-being of our economy more than any increased taxation could.