Apparently the marriage killed the entire comic book industry! Seriously, though. I think a lot of it has to do with changes in popular taste. When Superman and Batman appeared in the 30's, there was no TV, no internet, no video games, no significant competition for the entertainment dollar. Spidey, back when he was relevant, was one of the MOST popular icons on college campuses in the 60's. Now, no comic book character even comes close, and most people's connection to Spidey & Bats, arguably the two most consistently popular individual characters, is through the movies and vide games. Even the movies haven't had the same sort of impact on readership that the old Batman TV series appeared to have had on Batman sales back in the day. It's also hard to argue that comic quality is "worse" than in days past. The writing and artwork, if anything, are much better than in the Gold or Silver Ages.
I think what's hurt Spidey the most, as a character, is that he has ceased to be culturally relevant or to be edgy. Marvel feels like it can't take chances with the character, and I don't mean things like whether he's married or not, but whether he takes on issues of drug use, or abortion, or homelessness or other social issues. The Obama issue would have been the perfect time to make an interesting statement about the Iraq War or the financial crisis, but instead it has the impact of those old Hostess Twinkie ads. That might have made people think a bit, but instead Marvel gave us a banal story about the Chameleon and basketball. Of course, making something socially relevant is always risky for companies to do, as it doesn't translate to sales. If Pete turns out to be pro life and pro gay marriage, he offends everyone, and no one reads the thing. But if you're not pushing the social relevancy envelope (and I'm not sure that would drive sales any more), then you've got to be entertaining. It's there where comics may suffer, as they have to compete against the net, TV, video games, etc. for people's time and cash. There also appear to be more titles available today to read (not to mention the influx of Manga and the availability of free digital comics), so there's competition even among comics. The fact that the population has increased substantially from the 60's, but comic book readership has dwindled (at least paying readership, no one knows how many people pirate and stick with free net comics, so its possible over all readership is actually up), should tell us something. It's just not clear what.