In America, stores don't want the comics on the newsstands.
They don't make enough profit on them, so they usually only carry movie adaptations and stuff.
It's a pity.
Comics have to be available in newsstands, drugstores, supermarkets... Until they'll not be sold out from the comic shops, too, the sales will never be high.
When I see the Diamond top 300, I see Superman comics in the first positions (SUPERMAN and SUPERMAN/BATMAN, at least), and I'm glad for this. Then, I see the copies sold and I think "But americans are MILLIONS of people! How can the sales be so low compared to the population?".
My guesses for why comic sales are so low among Americans:
- The impression that comics are only for children (or "immature" adults). The stereotype of the "obsessed comic book nerd" (as seen on "The Simpsons") doesn't exactly help much...
- Comics aren't easily found here, as you noted---they foolishly opted to sell them starting in the 80's mainly through comic book shops, figuring "that's where the money is", when it just removed them from the public eye. Granted, newsstand vendors would rather stock "Maxim" over "Superman" given the choice anyway...
- Other competing forms of entertainment (video games, VCRs and DVDs, etc.) over the past 20 years.
- Reading rates in the US overall have slid off drastically in recent years, as newspaper sales have shown...
- Being able to see superheroes in other forms of entertainment; when one can play video games *as* Superman or see him on TV every day in various programs ("Lois and Clark", "Smallville", "Justice League", etc.), it might not leave one inclined to spend $2.95 for a 22-page comic about him.
- Speaking of that, comics are expensive for what one's getting. $3 for a 22-page booklet is expensive (though $7-8 for the occasional 80 or 100 page comic isn't overly cheap IMO either), particularly since they can't/don't tell a whole story in one, two or three issues anymore.
- Comics in the United States, unlike Europe/Japan, tend to be mainly focused on superheroes, with other genres (funny animals, humor, horror, etc.) having been almost completely killed off or suppressed. DC putting out licensed properties from its fellow Time-Warner properties isn't the same. Much as we like Superman, not everyone who might be interested in reading comics likes superheroes. (I understand the Disney Duck comics are quite popular in Italy, though they've had mixed sales here,as well known as Uncle Scrooge is).
Such limited genre focus also betrays that Americans still read comic strips in newspapers with some enthusiasm---millions still recall "Calvin and Hobbes" or "The Far Side" with joy (and buy their books and such).
- The trend in the past 20 years of constant retcons, revisions, "grim and gritty" stuff, crossovers, etc. has also driven away some long-term comic readers (as we're all familiar with). Not all superhero fans enjoy seeing Batman acting like a juvenile 15-year-old with Daffy Duck's ego... or at least, not *this* fan. (Keep expecting the current comics' Batman to start shouting Daffy-style "NO NO, I'M THE STAR, NOT THAT STUPID ALIEN!" any day now...)
- Related to several categories above, the lack of kid-friendly comics doesn't encourage new readers. Outside of the Animated versions of them (and "Scooby Doo"/"Archie"), comics tend to be adult-focused, with the companies seemingly pushing more and more toward this direction (with "Identify/Infinite Crisis" type stuff and the like). Can't imagine giving a kid a "real" Batman comic these days... yes, there's the animated versions of him, but still...
Thus, all of the above are reasons why Americans don't get into comics more IMO. Too bad...