Some of that is simple partisanship, though. In the worst days of the Bush admin when the disastrousness of the Iraq invasion was evident, and the scandals were pouring in, and his utter incompetence was evident to most, there was that 35%, all die-hard Republicans, who approved of him. Clinton is the #1 demogorgon for the GOP, and has been heavily demonized by the far left to boot, and nothing can really change that. Clinton’s approval numbers among Dems. are at the same level as Obama’s in 2012 at this point in the race, FWIW.
I wanted Sanders to win in part because he didn’t have all that baggage that really can’t be dropped, though if Sanders were in the race now, you can rest assured the hate machine on the right against the socialist would be in full force and his numbers wouldn’t be stellar either. There are partisan Dems who loathed Sanders who’d be likely to continue bearing grievances, just as some Sanders backers bear grievances against Clinton from the ugly primary.