It’s a direct result of having such a wide ranging field of candidates. Give the people a lot of choice, and they will choose however they see fit.
But also, bear in mind that a lack of popularity with Democrat Party voters doesn’t mean you can’t win the presidency. Obama in 2008 failed to win the popular vote in the Democratic Party presidential primaries - he needed the super delegates to get the nomination - but went on to become US president by a relatively large margin.
At this early stage, name recognition and proven experience in politics are the two key factors for each candidate’s popularity. The older generation of Biden, Sanders and Warren are clearly the beneficiaries of this.
For a young upstart is is extremely difficult, but not impossible, to break through. But it doesn’t look like there’s a new JFK on the horizon (yet).