It's actually not that hard; it's the difference between episodic fiction (TV and comics) and non-serialized (movies). You can only have a certain percentage of screentime devoted to introducing the characters, the situation, and the backstory. In serialized comics, you can spread this out over a very long period, as was done with the Rainbow Lantern concepts over about 3 years of storytelling.
The movie had to do it in less than two hours. See the problem? So because of that, the Green Lantern movie, even more so than most comic book origin story movies (and let's face it, most origin movies SUFFER for having so much time devoted to the origin stories) had to throw a lot of crap at the audience and hoped that they A) would keep and B) Give a damn. This is compounded by the fact that much of the Rainbow Lantern backstory derives in large part from the work of Alan Moore in his short stories - something a casual movie goer could not possibly care about (much to everyone's loss on that score).
Geoff Johns deserves some credit for Wold Newtoning up a unifed theory of Green Lanternhood and creating something that will make DC comics money in the years to come (though probably not so much money for him, but I don't have a lot of sympathy for this generation of copyright management, as opposed to previous ones), but it's not the most accessible story for a wider audience.
The GL movie just proved why that's important.