Because this would’ve happened;
http://www.vox.com/policy-and-politics/2017/3/14/14908272/trump-greenberg-democrats-neolib
That’s just one state, and one issue.
At the end of the day, I don’t think anyone can say definitively one way or another. I was pretty certain before the general that Sanders wouldn’t stand much of a chance as a self-described socialist. I’ll admit now, seeing how the election became anything but the ordinary policy debating and more of an all out war on character, that Bernie certainly would have a better chance than I initially thought. But I still don’t think you can underestimate the “goddam commie socialist” factor that many would have expressed, regardless of his positions on trade, labor, and the like.