One of the reasons for this, I believe, is the fact that both companies are cannibalizing their own stars and each other's works.
- Superman is often referred to as "the Last Son of Krypton." This used to mean that he was the only surviving Kryptonian when the planet exploded. However, he soon gained a dog (Krypto), a cousin (Supergirl), and a whole host of near relatives (Superboy, Mon-El, Power Girl, the Kandorians, General Zod and all of the Kryptonian criminals in the Phantom Zone).
- Batman now has Robin, Nightwing, Red Mask, Batwing, Batgirl, Batwoman, and even an entire network of pseudo-Batmen/women.
- Wolverine, for the longest time, was the only one of his kind. Then came Sabretooth, Weapon X, Daken, X-23, and Wild Thing, among others, who were all basically the same character re-hashed.
- The Hulk, a person who happened to have unique properties that allowed him to survive a gamma bomb explosion, has a number of duplicates. Outside of his large number of villains (Abomination, Leader, Gamma Dogs, etc.) with the exact same powers, there is now a Red Hulk, a Red She-Hulk, the original She-Hulk, Skaar (the son of the Hulk), Doc Sampson, A-Bomb, there was briefly a Hulk virus that turned everyone into a Hulk-like being. Even the Hulk himself has different iterations (Smart Hulk, Savage Hulk, Joe Fixit, Devil Hulk, Maestro, et al).
- Flash (Jay Garrick), Flash (Barry Allen), Flash (Wally West), Kid Flash, Impulse, Zoom, Jesse Quick, and countless others.
- Green Lantern (Alan Scott, Hal Jordan, John Stewart, Kyle Rayner, et al) led to there being a Green Lantern Corps and recently to there being multiple colored rings, each with a province over a different emotion or context and it's own Corps of people following that discipline.
I could go on with all the duplication of character ideas and concept, but I think the point is made: the companies are trying to give the public 'more of a good thing.' However, by doing so, they are actually watering down their own products! They are artificially inflating sales in the short term, but they are hurting their overall product line and long-term viability and growth at the same time.
This leads to a serious story-telling problem for the companies. Superman is the best at exemplifying this: a Kryptonian under the influences of a yellow sun is nigh invincible. Having two, four, or a thousand of him makes the original superfluous and means there are now a thousand nigh invincible, god-like beings at once. That's more than enough to solve all of the worlds problems either by helping human beings out or by destroying them (and maybe the planet) utterly, or something in between. It is simply too many. You already have to have incredibly complex and/or powerful things to make Superman's involvement worthwhile, then you add in a Supergirl, a Power Girl, a super dog, a Superboy and, at this point, DC Comic's universe doesn't NEED any other heroes-- these few can do it all. When you add up characters with similar power levels (Wonder Woman, Captain Marvel/Shazam, Martian Manhunter, Captain Atom, Green Lantern, etc.), the world wouldn't need a Batman or any other standard Hero, these Super Heroes would and could do it all... easily.
Each of the other characters is one who had a unique set of backgrounds, predispositions, and events occur to them that caused them to become the fictional heroes they did. Each time one of the companies creates a new version of the same character, they have to either mimic the original accident or situation (Supergirl was also blasted off from Krypton just in time, but was in a slower rocket ship; Kid Flash just happened to be standing in front of a similar chemistry table when another lightning bolt just happened to strike it and him, giving him the same speed-powers as his mentor; etc.) or they have to set out to make the same character in a new way (Daken is the son of Wolverine, so he picked up the original's healing factor and claws, and his hatred of his long-absent father gives him a similar level of anger; Mon-El is a Daxamite, and all Daxamites have all the same abilities as a Kryptonian when under a yellow star (like our sun) and a weakness to lead instead of Kryptonite; et al).
The other issue with uniqueness is that many of these characters inhabit more than one comic book. Currently, Superman can be found in: Superman, Action Comics, Adventures of Superman, Justice League, Superman/Batman, Superman Unchained, and the soon to be release Superman/Wonder Woman title. In addition, you have the Superboy, Supergirl, Teen Titans (Superboy), and Worlds Finest (Power Girl). All of the biggest stars of the comic book world have this issue: once the fans show they like the character, the comic producer puts them in steadily more and more titles until their line is saturated with the character. The Marvel comic universe would simply collapse if you removed every title that Wolverine and Spider-Man appear in regularly or star in. Same is true of Batman and his cronies in the DC comics universe.
As stated above, Superman and the Superman knock-offs are so powerful that a very few of them could affect life on Earth forever. When you then place him and his clones into so many titles, you have a situation where the over-saturation breaks the reader's immersion in the story. It simply isn't feasible that there are enough threats so constantly that require that level of power that often. Each one of those titles above has to have a threat of some sort in it;
When DC Comics recently revamped their entire line up and "started from scratch" with the New 52, they actually did NOT revamp or restart either Batman or Green Lantern. Both were having successful runs and had movies, so the editorial decision was made to simply renumber and continue with what had gone on. But this bad editorial decision has caused the "revamp" to slowly fail, as some parts were changed dramatically and other parts, anything that had to do with Batman or GL, had to stay primarily the same. The same happens at Marvel, too; you have one writer decide to strip Wolverine of his adamantium skeleton but another is writing an arc that absolutely requires it, and another is writing something that has him in space with the X-Men... which is the real Wolverine? Or are there now three running around the same universe in three different situations? Is Wolverine now Schrodinger's cat??
Marvel and DC Comics needs to take a look at their characters and titles and retract.
The other issue with uniqueness is that many of these characters inhabit more than one comic book. Currently, Superman can be found in: Superman, Action Comics, Adventures of Superman, Justice League, Superman/Batman, Superman Unchained, and the soon to be release Superman/Wonder Woman title. In addition, you have the Superboy, Supergirl, Teen Titans (Superboy), and Worlds Finest (Power Girl). All of the biggest stars of the comic book world have this issue: once the fans show they like the character, the comic producer puts them in steadily more and more titles until their line is saturated with the character. The Marvel comic universe would simply collapse if you removed every title that Wolverine and Spider-Man appear in regularly or star in. Same is true of Batman and his cronies in the DC comics universe.
As stated above, Superman and the Superman knock-offs are so powerful that a very few of them could affect life on Earth forever. When you then place him and his clones into so many titles, you have a situation where the over-saturation breaks the reader's immersion in the story. It simply isn't feasible that there are enough threats so constantly that require that level of power that often. Each one of those titles above has to have a threat of some sort in it;
When DC Comics recently revamped their entire line up and "started from scratch" with the New 52, they actually did NOT revamp or restart either Batman or Green Lantern. Both were having successful runs and had movies, so the editorial decision was made to simply renumber and continue with what had gone on. But this bad editorial decision has caused the "revamp" to slowly fail, as some parts were changed dramatically and other parts, anything that had to do with Batman or GL, had to stay primarily the same. The same happens at Marvel, too; you have one writer decide to strip Wolverine of his adamantium skeleton but another is writing an arc that absolutely requires it, and another is writing something that has him in space with the X-Men... which is the real Wolverine? Or are there now three running around the same universe in three different situations? Is Wolverine now Schrodinger's cat??
Marvel and DC Comics needs to take a look at their characters and titles and retract.