First off, Spider Man is a very popular character right now and has been for years. That helped the movies. Secondly, the films were very very good- Ebert even called the 2nd one one of the best films of the year. The reviews of both Spidey films were uniformly good. Casting was an important element, too- McGuire, Dunst & DeFoe are fine actors with pretty broad fan bases. And they were vigorously promoted, which helps.
In contrast, the reviews have been unkind to Superman Returns, and the precipitous 2nd week drop generally can be attributed to poor word of mouth (ie those who saw the movie are telling people they didn't like it). The reviews have been uniformly poor, with Ebert giving it 2 stars. Routh, particularly, has been criticized as being very flat & bland (the rumor circulating is that the only reason he was hired was his uncanny resemblance to Reeves...). The only actor of much note is Spacey, and even his reviews have been mixed in this one. Lastly, I don't think the studio did a very good job of promoting the film.