In many ways, it's not an easy "slam dunk" decision as to whether Biden or Bernie should be the democratic nominee. One important criterion is who will be more electable in the November general election. That's not really an easy guess either.
Some people think Bernie would be able to pull in more votes from angry lower income folks who are struggling and don't like the elites. They'd vote populist.
On the other hand much of the population of this country is still pretty conservative yet possibly a bit uncomfortable with Trump's wackiness. Crossover voters. Some Republicans and independents who Biden would be better at picking up. There's the Mitt Romney "Never Trumper" people and the constituency of Christianity Today that came out for Trump's removal from office. Still fairly conservative, but not really Trump fans.