For the same reason that he is the Democrat most likely to run successfully against Donald Trump in 2020, Joe Biden is the most problematic choice for the Democrats' presidential nomination.  Slow Joe, you see, spent decades as a prominent officeholder before the Democrats embraced crazy leftism as dogma.  And in the age of online video clips, social media, and email, his previous speeches contradicting the leftism currently demanded by the party's base are available for all to see.

A case in point is the current Democrat desperation measure: packing the Supreme Court.  Brent Scher of the Free Beacon remembers Biden's 1983 views on the subject:

Former vice president Joe Biden slammed the "bonehead idea" of packing the Supreme Court during a 1983 Senate Judiciary Committee hearing, saying the last attempt put into question the independence of the Court for a decade.

The remark didn't come during a hearing for a judge, but rather during debate over whether to allow President Ronald Reagan to replace members of the Commission on Civil Rights.  Biden opposed the nominated commissioners not because he viewed them as unqualified, but because he thought Reagan's takeover of the commission would damage its legitimacy.

He compared it to Roosevelt's court-packing push, which he called a "terrible, terrible mistake."

"President Roosevelt clearly had the right to send to the United States Senate and the United States Congress a proposal to pack the Court," Biden said during the hearing.  "It was totally within his right to do that — he violated no law, he was legalistically absolutely correct."

"But it was a bonehead idea.  It was a terrible, terrible mistake to make, and it put in question, for an entire decade, the independence of the most significant body — including the Congress in my view — the most significant body in this country, the Supreme Court of the United States of America."

The TV ads and blast emails write themselves.

Best of all, as a guy who loves to talk, there are plenty of other moments in committee hearings and on the Senate floor captured forever by C-SPAN, as well as other speeches that have been recorded and preserved, in which Joe contradicts current leftist orthodoxy.  That's why he is likable and why he appeals to more than the 30% of the public that is genuinely progressive.