Sanders is far more sane than any of the complete nutters running for republican nomination.
Christie and Fiorina are hardly nutters. Bush may be dull and insipid but on what basis he is a nutter?
They are all far more "mainstream" than Sanders.
Most of the polling has Sanders doing comparably well with Clinton against the Republican field. I think the 'unelectable' tag is unfair.
Hilary is toxic. She is extremely luck Biden and Warren aren't involved (yet). That Sanders is doing so well speaks volumes about how much even Dems dislike her.