Sure, Sanders was seen more positive in the primaries than either Clinton and Biden by independents. But other polls when asked if one would vote for a socialist which Trump and company would be running ads all over the place found on him being a socialist. Even from Sanders own mouth as he tells the world he is a Democratic Socialist. Only 45% of all Americans would even consider voting for a socialist.
If you compare them, Biden always does a bit better in March and April. I do think the general campaign would have driven Sanders numbers down when ads appearing of Sanders himself saying, "I'm a Democratic Socialist. Just my opinion.
It's high past time that we start electing Americans to congress and the presidency who put America first instead of their political party. For way too long we have been electing Republicans and Democrats who happen to be Americans instead of Americans who happen to be Republicans and Democrats.