There are the neoconservatives who fear Trump's appeal to isolationism and apparent willingness to cede regional hegemony to Russia and China. And then there are those who simply believe that, on his own merits, Donald Trump is an odious candidate.
Bernie Sanders has positive favorability ratings. He's the only leading candidate in 2016 with positive favorability ratings. If you care about losing to a Republican this year, then you should care about this fact.