Katherine Miller has them for you:
Biden has run twice; neither campaign went particularly well. He will turn 73 this year; Ronald Reagan was first sworn into office at age 69. The last three late entrants to a presidential primary — Rick Perry (2011), Fred Thompson (2007), Wesley Clark (2003) — bombed. An exception could possibly be made in the case of John Kerry, who declared his candidacy in September of 2003, but ran against Howard Dean and ultimately lost the presidency.
Each of those races accelerated the financial and organizational demands, as well as the scrutiny, of a presidential campaign, and now here we are.
Biden would enter the race against Bernie Sanders, who has proven to be an incredible small-donor fundraiser ($1 million in one recent day), and Hillary Clinton, whose campaign has already far surpassed $60 million combined between her campaign and super PAC. There is a finite amount of Democratic political money, and Biden has never been an expert fundraiser. One person estimated to the Washington Post that he would need to raise $30 million in campaign contributions to run in the first four states of the primary, and for a companion super PAC to raise three times that sum. During the 2008 presidential cycle, he raised $11.3 million.
He would need to staff a campaign, as well, in at least some collection of early states.
The clock is ticking, and organizations don’t build themselves while the potential candidate ponders his candidacy.