Joe Biden offers the most experience
Biden has the most experience in elected office of any major candidate this century.
Biden was previously Vice President to Barack Obama, meaning that to many he symbolises the Democratic politics of the Obama era.
Biden was the Vice President in one of the most popular administrations of all time. An "Obama-Biden era Democrat", he faithfully sticks to the ethos of the Obama administration. This is something that speaks to voters, as evidenced in polls where he is consistently the first-place Democratic candidate. Voters yearn to go back to a time before the madness of the Trump Presidency, with a candidate that has a sense of decorum, as both Obama and Biden do.
More to the point, Biden is not Obama. As an old, white man he does not inspire the same hope that Obama did in 2008. Electing Biden as the Democratic candidate does not, and cannot, replicate the Obama administration. Additionally, Biden is not even endorsed by the former President. Biden portrays a nostalgic idea of the Obama administration that does not actually exist. Post-Trump, Americans do not need "business as usual" from the Obama era; they need large scale change. The Obama era failed to the make broad change that many voters see as necessary, such as introducing single-payer healthcare or adequate climate change legislation.
[P1] Biden symbolises the Democratic party of the Obama era. [P2] Biden, as President, would bring about the return to the Obama era that America needs.
[P2] A return to Obama-era politics is not what America needs. It needs reform with innovative policies.