He was vice president for 8 years, and during a quite successful administration. Yes, I know VPs don't always do much, but Obama trusted him to take charge of a number of projects. And that's important-- Obama, whose judgment really is very sound-- trusts him. In fact, we know that's based on performance, because Obama was not a Joe fan in the beginning of their partnership, but became one as he worked with him.
Sanders isn't stupid or evil like Trump, and I believe were he president, Sanders would do his best for the country, in marked contrast to Trump (who doesn't give a sht). However, 30 years in Congress doesn't give especially the backbenchers much administration experience. And we now need someone who can make things work.
I'm not crazy about Biden-- I think he's too old (I'm 64, if that matters... I notice most of us near senior status tend to worry more about candidate age than younger people). I think he's too emotional. But his heart is in the right place, and he's got lots of hands-on experience with a crisis.
Oddly, we were right in 2008 choosing a man of little experience (but great temperament-- unflappable and thoughtful) to handle what was the last great crisis. We were lucky, as we were lucky in 1932, to get someone who was especially good in a crisis.
We don't have an Obama left in the race (though if we could somehow merge Warren and Buttigieg, we'd have someone of similar temperament/intellect combo), that we can count on to have the perfect instincts+insight to figure out what to do.
But we do have 1 person who has lots of experience and a reassuring manner, and we know he's not going to angrily crash the country into a bridge abutment like Trump might do. ("The EU was mean to me! I'm going to cut them off!"
Tried and true is a good guide for uncertain times. I feel better about Biden being the candidate (and next president
than I did two weeks ago, because I think the experience will help create confidence. And we need that.