During the primaries, the mild, establishment candidate muscles out a popular challenger from the left, securing their party nomination. This mild candidate is running against an obviously bad one fielded by the other party, self-evidently unable to do the job.
Aforementioned mild candidate then does not worry about creating an affirmative case for themself, and why would they? Their opponent is obviously bad. Much easier to say “wow, doesn’t that guy suck?” than to create plans to help fix the myriad of things that are broken.
This mild candidate does not campaign in battleground states (this time around, because it’s responsible and prudent), instead focused on fundraising from large-money donors.
Polling goes heavy in this candidate’s favor. There is just no way the obviously bad candidate could pull this out. The obviously bad candidate has a brush with his own hubris in early October, solidifying polling through the election.
Getting surprised on election day would be the definition of insanity, right?