In 2008, John McCain was behind; he needed to take some risks to turn things around. He was consequently persuaded to pick a figure who was untested on a national level as his running mate. The decision was a disaster: Palin proved to be totally unqualified for the job on substance, but she was also far more popular with the GOP Reactionary faction than he was, which meant she overshadowed him on the campaign trail on a daily basis. In the end, the risk was not worth taking.
Unlike McCain, Biden is ahead, but he still faces the same kind of choice. If he picks Warren or Harris, he will be running with someone with national experience–for better or worse, a known commodity. If he doesn’t, who knows?
To me, it’s a simple decision. Why take risks now?