They believe he's the most milquetoast they can produce. It's the same reason we have Starmer. He's a harmless, neo liberal, middle ground option meant to court floating voters.
Someone like Bernie Sanders, genuinely a genius, would have wiped the floor with both of them in that debate without even trying.
But the Democrats see him as unelectable, partly because he's easily tarred as a SoCiaLiSt, which is like a swearword in America, and partly because he wants to mega tax the billionaires that both parties rely on as doners