1) it's not like it's quantum physics. It's superheroes. A competent writer should be able to get you up to speed on a superhero pretty quickly.
Competent writers in hollywood aren't all that common. Look at all the cartoon and comic book adaptations that went over like a lead balloon. They VASTLY outnumber the good films 5 to 1, and that's being generous. Something like "Transformers" should be brain dead easy to do, yet all three films have been disasters. And bad as they were, GI Joe was inexplicably even worse.
2) the "target audience" of Justice League weren't superhero experts. They were kids.
This hasn't been true for decades. The target audience for DC and Marvel is the audience that grew up with these characters in the 70s, 80s, and 90s. Your average fanboy (much like your average console gamer) is in their mid thirties, and the stories have reflected this for quite some time. There is a world of difference between an Avengers story in 1988, and an Avengers story in 2014.
Origins aren't necessary to telling a superhero story. Most superheroes didn't even GET an origin until long after they had already established themselves as compelling characters to follow on their own.
Origin stories are necessary when films such as this one target an audience that does not follow the books, and even moreso when they follow other films that might conflict with what SvB is trying to do. Superman just got a film that works well as an origin, that's not needed. Batman on the other hand just had a film come out in which Bruce Wayne lost all his money and gave away wayne manor, wrecked his body almost to the point of failure before the film even started, and handed over the mantle of Batman to someone else. Who Batman even IS in this one is going to require some background.
And that's before getting to the B and C tier. No one knows anything about Aquaman, Wonder Woman, or Cyborg and those characters have more or less been confirmed. Green lantern just had a film that everyone hated that DC is probably going to want to retcon out if they want to use the character. Martian manhunter is EXTREMELY obscure to the general public. You can't just dump these wildly different individuals and power sets into the middle of a film with no explanation. Fanboys might be forgiving, but the broader audience isn't.
You don't have to BE Brad Bird in order to write a superhero movie without an origin in it. We're dealing with characters that are older than most people's GRANDPARENTS.
Which is a problem in and of itself. DC reboots these characters every decade and a half or so. The batman my father grew up with is vastly different than the batman i grew up with, and that one is different still than the one currently running around in the new 52. Superman? Same issue. golden age, silver age, bronze age and new 52 are all different characters. A definitive origin to reconcile this for those who have only a passing familiarity with the properties from childhood is necessary here.
The majority of the people who watched Avengers watched maybe one other Marvel movie before that. Maybe two. Probably both starring Robert Downey Jr. Somehow, that movie didn't trainwreck.
Hulk had two films, one that did $245 million, the other that did $263 million. Cap had one film, $370 million WW. Thor had one film, $449 million WW. And that's before we get into the Iron Man films.
All of these movies were available cheaply on DVD and netflix long before Avengers hit the box office, giving fans plenty of time to catch up. Marvel's reach with these films was a lot broader than you're giving them credit for.