I would point out that he did in fact run again, and in fact won the New Hampshire primary in 1968, but decided to call it quits after Gene McCarthy only lost by a few hundred votes in NH, IIRC.
Well yes, what else do they have to offer? That’s how our political system works: wherever the center is, one party’s gonna be to its left, and the other one’s gonna be to its right.
IMHO, the essential characteristic of a successful Presidential candidate is to be strong, to lead. This means being unapologetic about what s/he stands for, whatever that may be. The way you pick up centrists, I’m convinced, is not by moving towards where they are, but to decide who you are and what you’re for, and sell the centrists on it.
The prototypical exemplar of this philosophy, of course, is Ronald Reagan. The Carter crowd in 1980 was much more worried about running against a Republican centrist like Howard Baker, and not much worried about Reagan at all because he was too conservative to compete for the center. And we know how that worked out.
Another instance wasn’t a single candidate; it was a whole bunch of GOP House candidates. They stood on the Capitol steps and signed onto the Contract With America. That worked pretty well for them too.
Today, there are two factors that almost necessitate a similar approach for the Dems. First is that we’re in a much more polarized environment - there are fewer centrists in play, so the importance of mobilizing the people who are already on your side, giving them something to get out and vote for, is essential. And the second is that due to years of taking few stands, and then usually rolling over and playing dead even then, the Dems have less identity of their own than having an identity assigned to them by the pundits of the right. The best way for a party to regain its own identity is to unapologetically take some strong positions, and not back down when pressured.