What part of the mythos didn't they trust? They put 200M into what was, as far as surface details go, exactly like the comics, minus the veiny costume.The Endless summed it up pretty well, I think it goes like this:
When a movie exists purely by Executive Meddling to counter Iron Man/start a Universe/etc, then that movie's soul/core is the product of that executive. For Iron Man and the MCU, that person was Kevin Feige, who was mentored under Avi Arad and was in the room to learn first hand about every bad superhero film decision ever, so he knew from experience what to avoid. He's also an actual fan of the comics, so Iron Man had a compelling identity to Feige, and so the movie had a compelling identity when put before people like Favreau, and that identity grew, and then exploded when RDJ was cast and so by the time they do their final draft and start filming, everyone involved has a shared vision for the film. This is what happens with all good films, it's just usually the director bringing the vision to the studio and not vice versa.
Well, for Green Lantern, the vision was: check off all these boxes for a superhero movie, so we can make Marvel money. That's hard to rally around. Geoff Johns and the writers are actually the ideal people for that, but doing well in that doesn't really get you much of a movie, and not anything for other people to rally around. Geoff Johns does the ideal GL for silver age GL fans, but for people who don't already like Hal Jordan... meh.
Martin Campbell couldn't grow that, or get passionate about that. He was just checking the numbers off, which is, more or less, what he as hired to do.
Ryan Reynolds was, in a word, lost. The supporting cast did well because they're frikking award winners. They are all capable of carrying an indie movie all by themselves, if they haven't already. Mark Strong included. They actually could grow the concept of a check-the-boxes superhero movie, and they did, they elevated it. Without that supporting cast, this thing would have been closer to the 10% fresh mark and not so close to 30% fresh.
So, I guess I'm going about it a long winded way of saying the studio didn't ask for a good movie. It was never their intention. They just wanted a successful one, defined by focus-group-like properties. In the end they got neither because their stunts: name lead actors, award winning supporting cast, uber high budget, hot director, didn't come together. At all. Well, Ryan Reynolds and Blake Lively came together, but that's not what I'm talking about.

There was nothing about Martin Campbell or the script that excited Ryan Reynolds. There was nothing about the script or the character that excited Martin Campbell. So there was nothing about the movie that excited the audience, because there was nothing about the film that excited any of the filmmakers.