The way Sanders makes up for his favorability and crushes in the national polls is because he does far better than Clinton with Republicans and independents. Kasich has the highest net favorability of any Republican amongst Democrats at -4.
This is why those head to head polls are useless... the GOP has had 30 years to convince themselves that Hillary is bad... None of them know Sanders, but believe me, they won't like him any more than her when they're through with him. We have a sense of where her floor is... we don't know where his is. It could be a few points better, or 10 points worse.