Jesse Walker, over at Reason's Hit and Run, just posted an excellent piece making an obvious point. I'm not undercutting him here: part of why it's obvious is that he says it so well.
Here's the key part:
Sometimes they get more explicit, as when Sen. Dianne Feinstein said last week that Sanders' presence in the race has become "actually harmful," since it means Clinton "can't make that general-election pivot the way she should."
The "pivot," of course, is the moment a candidate stops pursuing her party's base and starts chasing the mushy moderates. The pivot is precisely what Sanders wants to block.
Summary of Feinstein's message to Bernie: You want outcome A. I don't. What you're doing will maximize your chance of achieving outcome A. So just stop it.
If Sanders quits, Clinton will be able to move closer to the median voter in the general election. That's Public Choice 101.
By the way, I have my own reason for wanting Bernie to quit. It's also Public Choice 101. If Sanders quits, Clinton pivots toward the median voter and then the Republican nominee, presumably Trump, doesn't have the incentive he otherwise has to go further left. Remember that in 1972, Nixon went left to compete with McGovern and Nixon proceed to "McGovern" from the left.
You might wonder why I wouldn't want Bernie in so that he can push his much-better, less-interventionist views on foreign policy. Simple. Because he doesn't. He mainly talks about the domestic economy and he's almost completely awful. The only candidate who's saying much non-interventionist in foreign policy is Trump.