people want candidates who advocate for good policy instead of starting from a compromised position and moving even further right from there
literally no one expected sanders to immediately deliver on a sweeping revolutionary socialist agenda if he were elected, but starting from the left can get you more of what you really want than starting from the middle
People want to hear simple solutions to complex problems from a person who is charismatic and engaging.
The vast majority of people don't care whether or not that policy is sufficiently socialist because the vast majority of people aren't socialist. They care if the politician has an understandable plan that might solve their problems that is agreeable to their own beliefs and values.
They also want to be able to put their hopes and dreams on their candidate who they identify with so that they can feel connected and important. this requires that the candidate be charismatic, inspiring, and emotionally engaging.
Sanders was certainly better than that then Clinton. However, Sanders being better than that than Clinton says nothing about pure left versus moderate left. All it says is that Clinton was a bad candidate. I am also skeptical that Sanders could have won because his beliefs and values don't align with a lot of the people who actually vote. Obviously, in hindsight it would have been better to let Sanders win the nomination considering we already know Clinton is a failure.