Biden enjoys a momentary lead in the polls basically because a great number of Americans haven't paid any attention to most of the candidates who have announced, and he and Sanders are probably the most recognizable Dem figures.
I suspect by this time next year Biden will be down the tubes, and I don't hold out much hope for Sanders either. Even though he's an avowed socialist.
They're both just too old and white. Even getting on their knees and groveling to the identitarians won't help them.