Part of it too is that the super hero fan base is just not that open to new characters. Marvel and DC are riding off the same crop of heroes it had by the 1960s. Every character in the Avengers was established by 1965. Batman's first appearance was in 1939, Superman 1932. I want to say of the current crop of leading X-Men the most recently created character was Kitty Pride who was introduced in 1980?
Anyway, the superhero fandom does not respond well to new characters. As a whole comics is a diverse fandom, but fans don't respond well to new characters. In the mid 1990s Chris Claremont launched Sovereign Seven a completely self contained team book in a corner of the DC universe. Despite Chris Claremont's renowned reputation from his decades long stint on X-Men (He's why Jean Grey is known as The Phoenix.) Sovereign Seven lasted maybe 30 issues?
The fan base does not support new heroes. Yes, George R.R. Martin had some limited success with WildCards, but overall the fan base is if not hostile to new characters, not supportive. I've been in rooms with Dan Jolley, who created Bloodhound, talking with fans about why they don't support new characters yet complain that comics constantly recycles itself. It's gotten so bad that DC brought back Wally West, who died in the 1980s, only to use him as a catalyst to reboot their entire universe.
Baen Books spent a lot of money and effort trying to establish original superhero fiction and found there really isn't a market for it.
If DC wanted to license a line of Robin, or Teen Titans YA books I think they would sell well enough. Or if Marvel wanted to a YA X-Men or Young Avengers novel line. But they haven't wanted to do that, and if they did that have a ton of established novelists who already write for them.
I'm not saying don't try it, just understand what the odds are.
I'm sorry, I don't mean to be dismissive, but this entire post is highly inaccurate and as a result, misleading. It's absolutely not that superhero fans aren't receptive to new ideas and characters....its that for the most part, publishers stick with the characters and properties that are known to make a profit. You're confusing cause and effect.
Not to mention your stats just aren't accurate. Take for instance the X-Men. In the current line-up of X-Men comics, there are easily a couple of dozen characters created in the three decades since Kitty Pryde who get just as much if not more page time than her. In the critically and commercially successful 'Wolverine and the X-Men' comic, fully half the cast was created in the past ten years, including Kid Omega, Oya, etc. The equally successful Uncanny X-Force stars Fantomex, a creation of the past ten years, and Deadpool, a creation of the nineties who has starred in several solo comicbook titles and featured prominently in the Wolverine movie and is being considered for his own spin off. These are fan favorite characters. Two of the characters in last year's X-Men: First Class movie only debuted in the comics in the past ten years. The last three X-Men video games have featured over a dozen characters from the nineties and the current century.
Claremont's 'Sovereign Seven' comic didn't fail because audiences don't want new characters, it failed because Claremont has been critically panned for almost twenty years now. All of his comics have done badly, including his return to the X-Men books, where he used all the old characters he'd once helped make famous. Sovereign Seven was quite simply just a bad comic. The characters were forgettable. Offhand I can think of a dozen superhero titles that debuted around the same time that are still thriving and with active fanbases. Gen13 - characters still active, rabid fanbase. Stormwatch, Wildcats, the Authority.....all hugely popular and critically and commercially viable.
George R. R. Martin did not have 'limited success' with Wildcards. Wildcards was one of many shared universe properties that all debuted around the same time in the early to mid-eighties. It shared shelf space with the likes of Thieves World and Shadowrun.....all of which have now gone the way of the dodo EXCEPT Wildcards. The 22nd volume of the Wildcards novels is due out this year....that's 22 novels in the same series since the first one debuted in 1987. That's not limited success. That's nearly unheard of longevity in any kind of fantasy or sci-fi prose series. There's a mere handful of other series that can claim that kind of longevity.
The idea that readers are hostile to new superhero characters is a fallacy much like the idea that comicbook readers can be polled on their opinions at all. There is a proportionally small but extremely vocal minority of comicbook readers who actively comment and post about comicbooks online. Some of the comicbooks that readers online seem to absolutely despise the most actually sell the most issues....because the vast majority of the people buying those issues just aren't joining the conversation online. Opinions online don't matter. Sales matter. And yes, a lot of new characters and books fail. A lot succeed as well. And quite often, there are other factors involved in why some books fail, and it quite often doesn't have anything to do with the characters' popularity.
You cited Runaways in another post....but the Runaways have been hugely popular with fans since their inception. Its just that after their creator left the title, the book changed writers every five or six issues, and none were able to find their footing. Runaways failed because of the WRITING, not because of the characters. The characters are still popular and frequently appear in other books, like currently where they're guest starring in Avengers Academy. You also cited the Young Avengers. The Young Avengers don't have a current monthly comic because their creator, Allan Heinberg is an active television writer who doesn't have time for a monthly comic and has an agreement with Marvel to be the only one to write them. Those characters are also hugely popular, and one of the most common questions Marvel hears is 'where are the Young Avengers showing up next?'
I don't mean to nitpick, but the idea that a superhero book is any harder to make successful than any other genre book is just absolutely untrue. The absence of a wildly successful superhero prose novel isn't an indicator of anything other than the fact that one hasn't broken out yet, much like dozens of other 'niche' premises. Good characters are good characters, its that simple. If a hundred fantasy books can do their own version of the chosen one sets out on a quest to save the world premise, there's no reason a hundred superheroes can't derive from similar beginnings.
The only problem with most new superheroes was mentioned tangentially earlier in the thread. The thing is, so many people focus on the gimmick, or the name, or the power....the flashy, superficial aspect that may be the first thing readers see about the character, but can't BE a character, in and of itself. Too many superhero writers forget to write a character first, a superhero/powers second. Spiderman isn't a beloved character because he has the powers of a spider....its because of his trademark theme of with great power comes great responsibility. Superman isn't Superman because he flies....its because he's the last survivor of an alien planet dedicated to defending his adopted world. Batman isn't cool because he dresses like a bat....but because he's a badass vigilante who was traumatized by the murder of his parents and set out to take justice in his own hands.
Figure out why your character is a hero in the first place, what makes them do it, what their higher calling is....THEN worry about their powers. That's what sets the memorable heroes apart from the crowd.