That wouldn't kill the movie's box office potential. There were many terrible movies that made more money come on.
It was a case of people not being interested in Superman. The public has moved past him, sort of like Captain America and other cookie cutter heroes. Obviously Cap has made huge leaps as a character recently, but the public still sees him as that 60's goodie two shoes. Meh
Spiderman transcends generations in ways Superman doesn't. Consider the slight backlash Superman Returns got for revealing Superman's illegitimate son lol. People expect Superman to live up to a certain morality, one which we don't expect of ourselves. Spiderman, on the other hand, is far more human and "regular" than Superman. He has to pay his bills, find a job, compete with the jocks, etc.