Said who, the media? Trump always had a chance. And on the eve of the election, he had over a 30% chance of winning based on the data (nate silver). The data was and always will be correct. Clinton won the popular vote by 2%, just as the polls showed. Trump won PA, MI, FL as the trend in the polls indicated. The one and only surprise that the polls got wrong was Wisconsin. And you can tribute that to them using previous election models to simulate turnout. You know, you need to actually look at the data and looks at guys like Nate Silver to get an idea - not listen to stupid media pundits.
And young people just don't show up. Period. Biden leads the most reliable pack of voters. He also does far better with non-white voters, which is key in the important states.
FYI, Clinton was an awful candidate. Qualified? Very much so. Likeable? Genuine? Not in a million years. And yes as a woman, she did face extra challenges. Certain demographics that always vote democrat, did indeed go for trump or stayed home (in very key areas, ex. south Florida) because they did not want a female leader. That's a cultural thing for them.
And again, Biden crushes Trump in every poll. State level, nationwide, 3ways, etc. Bernie is in toss ups in most polls vs Trump, which is scary for democrats since Bernie has never faced true attacks.
A general election with him will be about socialism vs capitalism and that will mobilize any and all voters who aren't left wing. Hell, I very strongly dislike Trump and want him out of office asap and would likely vote for him if it was him vs Bernie.