Clinton is trying to smother Sanders with agreement.

David Axelrod, who ran Obama’s winning presidential campaign in 2008, made a shrewd observation on Twitter:

It’s exactly right that as the underdog and challenger, Sanders is trying to differentiate himself from Clinton, to give people a reason to vote for him. Clinton, by contrast, is trying to suggest that she and Sanders agree on goals, but she’s more adept at achieving those goals. Clinton also doesn’t need to attack Sanders directly because she has many more surrogates in the party who will do that for her. 

“We’re in vigorous agreement here,” Clinton said at one point. When Sanders suggested he and Clinton differed on expanding Social Security, Clinton responded, “I think it’s fair to say we don’t have a disagreement.” After outlining her policy of focusing on low-income seniors, she ended by saying, “We have no disagreement about the need to buttress Social Security, get more revenue into the program. ...I think we’re in vigorous agreement here. ...We’re going to end up in the same place.” 

Sanders might have tried to provoke an argument, but Clinton was set on extinguishing his attempt to seem like an alternative.