I had a thread about this a while ago but I'm bringing this up again:
It seems many who don't support Sanders base their reasons partly on their doubt that any of his proposal and plans would pass the Congress and actually get implemented.
OTOH, many who don't support Trump base their reasons on their fear of the consequences of his proposed plans (immigration, nuclear button, etc.).
So Sanders can't get his plans realized, while Trump can.
(And of course, Clinton appeals to every side, even conflicting sides.
)