In my opinion Marvel is doing this all wrong. They are spending all their time stretching out the ranks of teams like The X-men and The Avengers into more teams that are really variations of the same theme. They should bring back the days of the 70's and 80's when there were other teams like The Defenders and The Champions.  
 
 

Well let's look at it like this.With 3 Avengers teams,more characters can get some burn in a mainstream comic.It also makes sense that their is 3 teams because of the circumstances.Norman's Dark Avengers are the official Avengers by law.Pym's Mighty Avengers are Tony's Mighty Avengers after the Secret Invasion and Ronin\Ms.Marvel's New Avengers are Cap's New\Secret Avengers after Civil War,Initiative,and Secret Invasion.This is the thing.They all want to be Avengers because they were chose for it but they won't join with Norman because he's evil and they won't join with Tony\Pym's Avengers because they were enemies during Civil War & Initiative.It makes sense.However you know with the Mighty Avengers and the New Avengers both being fugitives of the law essentially putting them on the same side and the Dark Avengers being composed of villains.This will not last.I predict a another Heroes Reborn...a re-definition of what it means to be a hero. 
 
Let's face it.Other teams in Marvel will never be as popular as the X-men or the Avengers and this is why those books are cancelled.New Warriors just got cancelled for sales,Heroes for Hire before that, and i'm positive the same happened with the other books.There is really no need for all these teams to have books.They could really use the initiative book to showcase them all and then give them mini-series periodically.Or they could put those teams together in another book and make it a "Marvel Comics Presents" type thing.Three different stories and teams in one book.

" we're in an era and time of not bringing things back but instead improving on them or scrapping it and starting from scratch... there's only so many ideas you can come up with b4 u kill what it is ur trying to re-create or improve. "

" @Statman said:

" we're in an era and time of not bringing things back but instead improving on them or scrapping it and starting from scratch... there's only so many ideas you can come up with b4 u kill what it is ur trying to re-create or improve. "

It seems to be working well for DC comics. Look at how well The JSA comics are doing. That's because of the time an effort being taken to make the team and comic as good as the JLA. With more and more villains in Marvel teaming up, it would be a great time to re-introduce or re-invent these great teams. Spread the wealth and mix it up a bit. Instead of doing the same old thing. It's getting pretty predictable. "

I know what you mean.  We got to see the Defenders not too long ago but they were being written by Loeb and we haven't seen the Champions since WWH. But one thing you gotta keep in mind is that with the 50 state initiative many of the classic team names are being used for certain states. 

" @Vance Astro: I would say that the outsiders or the titans are more like Marvel's X-men, but that's how i see the characters. The JSA are made up of DC's less popular characters, though they are still somewhat known. But DC is working hard on trying to change that. For example, most people know about Hal Jordan and even John Stewart, Guy and Kyle. And given a preference most would rather read about their exploits than Alan Scott a few years ago, but now he is growing in popularity and those who didn't know about him are want to know more now. "

" @Vance Astro: The Thunderbolts and the Intitive are Avengers connected books. They are like The New Mutants to the X-Men and if the Brotherhood started a series. And you really can't see the Secret warriors and the Agents lasting do you? My point is that there is a world out there that the Avengers and X-men are not a part of or don't have to be. Marvels' world can be larger than what is shown these days. I mean having everyone in one city is a old concept. Why would the villains stay there? Marvel needs to takes some hints from Dc and stop riding their one trick pony. "

The Defenders get revived about once every five years, and it fails each and every time.
 
Alpha Flight/Omega Flight? Same deal. I can't even remember how many Flight relaunches there have been, now...
 
The Champions ALMOST made a comeback, which ended-up being "The Order". See also, "The Failure". I believe that the solo members of the group either belong solo like Herc and Ghost Rider, or belong with the teams they're on (Iceman, Angel, Black Widow). They had their time in the sun as a group...moving on...
 
The Agents of Atlas deserves to be up there with Avengers and X-Men, but probably never will be because not enough people are giving it a chance. Hopefully that will change once they re-launch as an "X-Men" back-up feature, but we'll see...
 
MI:13 was another group with unlimited potential, but again, fans are just too scared it seems to try something new, as it was canceled after a year. 
 
So to answer your question as to WHY there aren't any other teams? The fans are the reason, apparently. People speak with their dollars, and the majority don't WANT to branch-out; they'd be happier with another X-Book, something familiar, than risk something new. It sucks, terribly, but until that train of thought derails, we're all stuck with it.

"The Agents of Atlas deserves to be up there with Avengers and X-Men, but probably never will be because not enough people are giving it a chance. "

" @RaptorFratBoy: I don't think it's the readers but the writers who are assigned these books. They never get the tip tier people. Just the new-bees, the never-was and the has-beens. Even the bad writers who are some how popular (Jeph Loeb) don't get the titles. Imagine how well The Champions would be received if someone like Bendis was working on them. Or Millar on the Defenders. "