Favorability polls really aren't a good indicator, the general public knows almost nothing about Sanders and he hasn't been attacked by the GOP for decades like Hillary. In fact the Republicans have been pushing hard for Bernie's nomination, Karl Rove is literally running ads promoting Bernie, Breibart and DailyCaller pump out pro-Bernie articles every day. Even FOX is pushing for Bernie as the Democrat nominee.
If Bernie does by some miracle get nominated by the dems (currently Clinton is leading him 502 to 71), the GOP will go hard in attacking him with the following and his favorability will tank: