It’s not surprising at all that Bats is outselling the league. People will buy Batman books even if their crap.
Oddly, I have high hopes for Justice League of America. I think Johns might be more enthused for it.
All cruel people describe themselves as paragons of frankness.-Tennessee Williams
As others have said, Batman is definitely the reason it's not the best selling title. The whole Death of the Family arc has made a pretty big deal, so that's bound to make it one of the best sellers (in this case, it is the best seller). The only reason I'm reading Justice League is because they made Captain Marvel (I mean "Shazam") a team member. I do like that they introduced Ocean Master, and made him a pretty big threat. One thing I never loved about Aquaman was that he didn't have any great villains except for Black Manta. Now they're introducing Ocean Master and not only making him an Aquaman villain, but a Justice League villain as well. This also kind of relates to the whole not number one thing, though, because some newer comic book fans probably would want to see villains they know or something like that. I like it, though.
Well it's DC's second best selling title so it's hardly doing bad, but having said that I'd say the reason it hasn't held onto sales as well as Batman is simply because it's not very good. It has fan favourite characters but does a piss poor job of showcasing them. The first arc was a mess with little to no story and just lots of random fighting, the second put the League into secondary character poitions in favour of Steve Trevor, and the newest arc is a definite improvement but is missing Green Lantern and Flash, and still shows a fractured League that squabble like children and relies on them acting like idiots to move the story forward. It has not been a great time for them.
While Batman has kept up a pretty consistent quality in it's writing and art. Personally I'm not enjoying Death of the Family, and I felt like Court of Owls went on too long, but I wouldn't say that it's anywhere near the mess that Justice League is.
Batgirl, JLB 2.0, Letter 44, Mr. Peabody & Sherman, Powerpuff Girls, Rocket Girl, Swamp Thing, The Ultimates, Ult. Spider-Man
Inhumans, Legenderry, Tiny Titans, Tomb Raider
I mean, logically, there are people buying Superman, Batman, and Wonder Woman comics but not buying the comic Superman, Batman, and Wonder Woman in it.
The DC relaunch was successful and was executed in what was most likely the best way it could given restrictions we wouldn't know about. No, your idea wouldn't have worked. Now move on.
More specifically to the OP I would assume that despite the book featuring all the major players, it is not all the major players fans that want to follow them over into a book where they have to share panel space with just as large personalities. Like currently Aquaman fans will be quite happy to read it, but Barry and Hal fans have no reason to unless they are also fans of Aquaman.
While there are alot of people who will buy this book purely because of Johns being the writer, even they are limited.
In regards to new/unrelated readers, well first of all they have to pick it out of dozens of other titles and right now a book like Avengers has a much bigger name recognition than Justice League. To new readers the name of Geoff Johns, Jim Lee and/or Ivan Ries will mean nothing. Perhaps the 7 for 1 idea of the team book sounds great on paper for a new reader, an entry point for them to the world of DC, but it comes with an intimidation factor of a new reader having to figure out not 1 but 7 characters that dont all get a fair share of exposure from issue to issue.
To older, more cynical/critical, readers however, there is the recognition that Johns is at his best when writing solo characters and his team-books normally are serviceable but nothing exceptional (I know some praise his JSA run, but I haven't read it...but I have read his TT run and that amounted to almost nothing). Theres how everyone's acting different in JL to a greater or lesser degree when compared to their solo books that can be off-putting or annoying. And finally theres the problem of the stories themselves not being terribly good save for a few cool moments.
My work: http://www.fanfiction.net/~outside85
Because Geoff Johns, my reason for not buying it at least not for full price.
Pull-list: Action Comics, Batman, Superior Spider-Man, The Flash, Superman, Batman/Superman, Superman/WW, Superman Unchained, Earth-2, Harley Quinn, Astro City, Detective Comics
I can sum up most of these two pages with one simple quote from Dr. Sheldon Cooper "I'M BATMAN"
Thanks to The Movement I don't miss Teen Titans that much anymore.