There are a couple of reasons that The Avengers worked that JLA does not have. Regardless of how WB plans it.
The Marvel universe has been integrated in the comics from the beginning. With crossover appearances right away. Because Stan Lee wrote all the books. DC was not like that for years. The JSA and the JLA rarely interacted in their own books. Just Superman and Batman for a long time.
As a result with DC it seems more forced. because a lot of DC's characters where based on the Superman archetype in some way. Only Batman is completely different from him. So it's not surprising that those are their two most popular characters.
While The Avengers are completely different from each other. Which offer great contrasts and conflicts. Due to mostly being from the same team of Lee and Kirby. They wanted each book to be different from the other to be more interesting for them to create.