If Sanders would be nominated, he’d have to swing all the superdelegates to him, overturning the popular vote and pledged delegate totals. The only argument he has is his superior electability.
Sanders electability argument hinges on his superior general election numbers.
From Vanity Fair, here’s the plan from the Sanders campaign manager Jeff Weaver.
“’If you can’t come to them and say, we won the popular vote, you have to honor the will of the people . . . how can you flip them after the primaries?’ he asked.
‘Well, because they are going to want to win in November,’ Weaver insisted, arguing that even if Sanders wasn’t a winning candidate among the Democrats, he would perform better against Trump in the general election.”
If you’re a Sanders supporter and want to help him get nominated, it follows that you should want to make Hillary Clinton’s general election poll numbers worse than Sanders. You do that by answering pollsters and telling them you’d vote for Sanders in the general but not Clinton. As a Sanders supporters, even if you really intend to vote for Clinton come November, claiming you won’t is the only way to help Sanders get nominated. This is one more dynamic that could be suppressing Hillary Clinton’s polling.