Weren't they historically bad through most of the 60s and 70s? I just always assumed that was the reason.
I actually (admittedly) think it's been a tough place to play when Fry changed the culture and they were decent in the 80's. It's probably a little tricky to focus on the game when home fans are throwing cigarette butts at you.
Add in that your program was the decent one when the other (ISU) was in bad shape and the big fish/small pond effect takes place.
'Historic' Iowa football is the equivalent of the person who started for a few sports teams in small-town high school and still talks about it 20-30 years later, which is what a lot of their fans are as it is.