It's weird that they are still polling this, but also we had elections in multiple states across the country last Tuesday and nowhere did Bernie Sanders come anywhere close to winning 33% of the vote.
The question put out in the last few surveys has been along the lines of, "Who would you have preferred to be the nominee?" (from a choice of these two only).
Yeah, a lot of people who would have supported Sanders aren't bothering to vote in the primaries anymore, somewhat understandably.
Of course, some of them didn't bother to come vote even when he was still in the race which is a big part of why he's not the nominee...