Hillary Clinton has been declining slowly lately, and even with all of the media's support, she has not been gaining support, only lose them. She is the absolute worst candidate to face Donald Trump, because she has way too many weaknesses that Trump can pick on. Bernie, on the other hand, will absolutely demolish Donald Trump, and from the way things are heading right now, he is likely to defeat Hillary Clinton with this uprising he's been on lately.
Hillary Clinton is the more moderate of the two, plus she can play the gender card ("look mom! No dick!"). Sanders' key demographic are people too young and stupid to vote who will come to their senses by the 2020 election. The country has more GOP supporters in this election and Trump has enough supporters that he will win. Case closed.