Well, they weren't presidential. Often, we vote not for intellectual reasons, but out of gut instinct and first impressions. Gore likely lost in 2000 because of his effeminate accent. Bernie is too cranky, Warren is too academic. Both seem too old--as does Biden, but there was no other choice to get rid of Trump.
The problem is the liberal humanistic principles that are essential to early-modernity are misunderstood as radical and socialist in the US. The Cold War rhetoric has distorted the issues, so American liberals and progressives are confused about their identity. The left wing is easily painted as radical, socialist and un-American, so you need a winsome representative to counter that framing, not an annoying, hectoring or lecturing one.
But the underlying problem, I'm sorry to say, is that Democrats are too urbane and hyperfeminine to be able to do politics well, not when they're up against a more masculine (savage, psychopathic, unapologetic) opponent. Trump could sell a fridge to an Inuit. Democrats couldn't sell Heaven to a born-again Christian.