In his emotional speech announcing he is not running for president, Joe Biden basically blamed the clock:
I believe we're out of time, the time necessary to mount a winning campaign for the nomination. ... As my family and I have worked through the grieving process, I've said all along... it may very well be that that process, by the time we get through it, closes the window on mounting a realistic campaign for president, that it might close. I've concluded that it has closed.
One can certainly sympathize. It's hardly uncommon for sitting vice presidents to seek their party's presidential nomination, and if Biden were going to do that, it would have made sense to move earlier. The great tragedy he and his family suffered earlier this year surely derailed that to some extent.
But really, time wasn't Biden's problem. His problem was Hillary Clinton. Or, more accurately, his problem was that Democratic Party elites had already committed to Hillary Clinton.
By just about any measure we have short of an actual election or convention vote, the Democratic Party has made incredibly clear that Hillary Clinton is its presidential nominee for 2016, and it's been broadcasting that signal for more than a year. Whether you want to look at elite endorsements, fundraising, polling, Super PAC support, or anything else, it's long been obvious that this wasn't going to be much of a contest. Most high-quality Democratic governors and senators heard that signal and figured this wasn't the year for them to run. That's why Clinton's only major opponent in the Democratic race right now is a self-described democratic socialist who's not even a member of the party.
Biden clearly heard this signal, too. As the sitting vice president (and by all accounts a pretty good one), he surely gets some deference, but that clearly wasn't enough to translate into a presidential run.
As I noted here, Democratic party elites have already vetted Biden. They thoroughly considered him in two previous presidential runs — 1988 and 2008 — and found him wanting. There was little reason to believe they'd rally for him this time.
Now, if Hillary Clinton had decided several years ago not to seek the presidency, and the Democratic field consisted of a handful of governors and senators with decent records but little name recognition among the public and no clear support by party elites, there would certainly be a rationale for a Biden candidacy. Lack of time would not have been much of an issue. Through his ties and, perhaps, with the backing of the White House, he could have quickly assembled a credible campaign and a decent field staff in New Hampshire and Iowa. It would have been a scramble, but hardly an impossible task.
But that's not the environment Biden faced. He couldn't gather the party behind him not because it would take too long, but because the party was already gathered behind someone else.