He knows the Democratic voting habits, and is focusing on the large bloc of voters who ignore ideas and solutions they see as too far to the left. At the same time, though, he is also presenting positions that echo the left's ideas on the direction to follow. He is proposing an expansion of Medicaid to act as a way to supply healthcare to those who fall outside the normal employer provided plans. He is calling for a $15 minimum wage, which is exactly what people like Sanders have been calling for.
He's also focusing on defeating Donald Trump and undoing the harm Trump has done. That's very important to a very large bloc of Democratic primary voters. Biden is a consummate Democrat, with a long history of leadership. His close association with Barack Obama doesn't hurt him, either. People remember Obama hanging the Presidential Medal of Freedom around his neck as a surprise. They remember the close friendship of Obama and Biden.
The primary race is not a debate team contest. It is a contest of politicians and leadership. A large number of Democratic voters are terminally weary of political drama. Joe Biden represents a less dramatic and less contentious political reality, and that is very, very appealing to many voters - perhaps a majority of voters.
As a reliable primary voter who always votes in every primary election, I have seen who is there to vote in my precinct. I've also worked at a polling place a few times during a primary election. Most primary voters are the sort of solid, Democratic people who vote out of a sense of responsibility to participate. Generally, they are older, in my experience, than the median age of the local population.
Who turns out to vote in the primaries will largely determine who is the winner. Right now, Biden has a solid lead in current polling of likely primary voters. Very solid.