I think part of the reason why more people support Biden over Sanders is that Joe Biden actually has experience as a Vice President while Bernie sanders has only been a senator.
Actually they both have a bit more experience than that. Biden served as a Delaware senator for 36 years before becoming vice president, but he also served (at various points) on the Foreign Relations Committee, as the chair of the International Narcotics Control Caucus, and as the chair of the Judiciary Committee. Sanders has only been a senator since 2007, but also spent 16 years as a representative of Vermont’s At-Large District of the House of Representatives, and he was the mayor of Burlington for 8 years.