I mean, this has Heller as less popular than Trump and Trump lost Nevada to Clinton. Why would Heller then be able to beat a halfway competent candidate tailored to the state?
For the same reason so many people think Donnelly and McCaskill are favored: Incumbency.
It's ridiculous, and I agree with you that any halfway competent Democratic candidate will beat Heller. The only reason I'm not as sure of the outcome here as I am in IN and MO is that NV isn't really THAT Democratic, even though it leans blue obviously. The race is definitely Lean or Likely D, though.