To me, the question "Why didn't you do it like those other people did it?" provides its own answer. The fact that Marvel did it one way is the best possible reason for DC/Warner Bros. to do it a different way.
The problems with that idea are many:
1) WB/DC have no established track record of success to inspire audience confidence in the project to generate buzz. Outside of Supes and Bats no DC project has been considered a substantial success either economically or critically. Indeed, it's most recent attempt is widely held to be a resounding failure
That means they either have to tentpole the film on Supes & Bats (with everyone else being essentially walk-ons in terms of story focus) OR they have to run the huge risk of trying to draw people to these characters whom (if we are being brutally honest) have little or no recognition in the eyes of Joe Buttinseat.
EVERY major character in Avengers already had good name recognition and posititve buzz from a prior movie success (with the possible
exception of Hulk). Even Hawkeye's extremely limited showing in Thor drew a great deal of critical and fan approval.
2) Marvel "went big" with Avengers. By your reasoning, DC should "go small" with Justice League, which will leave the audience wondering why all those super-powerful "superheroes" are doing very little in terms of daring-do.
That would be X-Men 1 all over again: no "super" in the superhero movie, leaving the fans of the "BANG! POW! ZAP!" unsatisfied.
On the other hand, they can hardly "go bigger" either. In terms of raw power, the JL has the Avengers dead to rights. There's a reason why they call the Seven a "pantheon". "Going bigger" runs the risk of taking JL into that "cosmic zone" of superherodom that the perceived failure of F4 1 and 2 and Green Lantern seem to indicate that the casual audience cannot relate to.
So you want DC to take a bunch of untested
characters, portrayed by unproven
actors and try to craft a story that will appeal to Joe and Jane Buttinseat but also
do justice to seven "gods on Earth" to appeal to the fanbase..that's a lot to pile onto one film with maybe a $250 million or so budget.