Well, 1980 polling and 2016 polling are not comparable. We've come a long way, baby.
I remember that election, though. I voted for Reagan. I also knew that Reagan would win. All I had to do was watch the news. The economy was out of control, with double digit inflation. Complaints by citizens everywhere. Even Carter's own party had tried to dissuade him from running for a 2nd term.
So I don't know what went wrong with that polling. Everyone seemed to know that Carter wouldn't win, as I recall, although no one suspected a landslide by Reagan.
I just looked it up. Wikipedia says the last poll for that election was Reagan 47%, Carter 44%. So the poll had picked up Reagan's last minute surge from 39% to 47%. In the end, it was the economy. And a third party interloper, who took quite a few votes from Carter but none from Reagan.
Not a great comparison, though. The times were different, polls were very different (they're better, now), and Carter was an incumbent, the economy was horrible, Carter's own party didn't support him fully and he even had a party member try to oust him (Teddy Kennedy). So, very different, in my view.
But I understand how Trump supporters might want to view it as similar.