I think there are two different issues at the core of this, and American comics lose on both accounts:
As everyone knows/has said, comics are a very expensive medium, in terms of dollars per time entertained. An average comic is about a dollar every 1-3 minutes, if you read at a standard pace.
I'd base the standard for entertainment value today on the DVD, and that's about a dollar for five minutes of high-production value entertainment. [And I buy a DVD knowing that at least a half-dozen of my friends and family will also enjoy it -- boosting its value even more... which is never true of comics.]
What's changed this year?
Manga and trades now offer improved value: $10-20 for a half-hour to two-hour read. That about equals the value of a DVD. [Except that, even as a comics lover, I'd be hard pressed to argue that minute by minute the entertainment value of a movie is only equal to that of even the best comic.]
Monthly comics value declined. And worse, the TV-equivalent of the boost in ad pages would be like being forced to watch a 30-second spot... every thirty seconds. Who would watch TV at that rate?
It always amazes me when any comic reader says that today's content is at all diverse. Yes, there are one or two books in almost whatever niche you can name -- but most comics are distinctly well within a niche genre: heroics and horror, widely speaking.
What's always even more amazing to me is when independent creators complain of limited sales -- when what they are offering is of such narrow interest that even if the subject matter was presented as a free TV show on a major network with great production values -- it would be canceled due to low ratings.
[Think about how many "genre" TV shows are canceled -- especially this season, where, in the wake of "Lost" everyone thought there was a large "mainstream" audience ready to accept more stories of aliens, ghosts, or spaceships.]
[Let alone the fact that most indie comics are poorly written and drawn by any standard of professional entertainment.]
Read through a monthly list of new titles -- and I mean just that, the titles, the names of the comics -- and just by the titles alone anyone can see that the books appeal to an extremely narrow [if not non-existent] audience.
This perception is enforced even by most comic news that is covered in the mainstream press.
Look at the art and the title of the series that is the first to be distributed through iTunes: doesn't that just scream "aimed only at superhero comic readers?"
In any event:
What is mainstream?
An earlier writer used the Grisham example of a mainstream book. Aside from the fact that most adults don't read books at all, let alone suspense books about heroic lawyers, Grisham's sales would meet many people's definition of "mainstream" entertainment -- but by that sales meter, so would books and movies about teenage wizards and fightin' elves.
So does mainstream mean mass-market sales figures, or does it mean "non-geeky" subject matter?
In the latter, there are very few comics out there that could be grouped with such stories as Grisham based on story content alone: an identifiable protagonist, in the real world: no powers or magic.
A lot of Warren Ellis’ recent work fits the bill of non-fantasy action -- Down, Red, Fell, Cross. [Jones skirts into sci-fi.]
Apart from serialization [a whole 'nother issue with pros and cons] -- Do these stories appeal to a widespread readership?
Are the trades a good entertainment value?
I'd say yes to both -- which leaves the question of why aren't they mass-market successes?
Is it too soon to call on these experimental comics?
Is it just distribution?
Is it public perception of "comics" and what that always implies about story content?
Or is it the most basic factor: arguments about newspaper comic strips aside, most people don't like the comics medium -- at least for "long form" stories of more than a few panels.
Many seem to define all manga as "mainstream" just because it is outselling superhero comics... Even though 99.9 percent of American adults wouldn’t read manga because of its primarily fantasy story elements -- or the simple fact that it’s comics. Let alone the exaggerated art style.
The content of American comics is a narrow niche -- but I'd argue that the medium, whatever stories it is telling, will also always be a niche. Never mainstream.
Apart from implying that no comic creator will ever be as wealthy as Grisham -- what's wrong with that?