That made no sense as Mondale was the vice president to Jimmy Carter who was very unpopular and was voted out in a landslide. So you would think that they would try to go to someone who wasnt associated with Carter to run vs Reagan in 1984. Yah they would still lose big but it would be more like 1988 then 1984.
While he was Carter's VP, he still had strong ties to the Democratic coalition. I think that coalition had buyers remorse after the failed Carter presidency (they never trusted him but fell in line in 1976 against Ford) there was a feeling to return to business as usual. That combination of old & stale Democratic ideas vs. a "morning in America" of Reagan combined to doom Mondale.
