No one should be surprised that Hillary has such high numbers. I do wish that the other candidates spoke up more and sent jabs against their rivals; with the exception of Sanders who's been featured in the media and has a small grouping of dedicated admires.
At some point, Sanders is going to have to launch some attack ads against Hillary. There's only so far you can get with a positive campaign in this day and age.
Why should he do that if his goal isn't to win the nomination?
Well, sure, if he just wants to get a friendly plank or two in the platform, speaking time at the convention, and maybe a cabinet position, he doesn't need to. But I'm assuming he actually wants the nomination.