John Carter failed because people are over the trope of nearly indestructible hero. The book inspired the Dork of Steel. Which is another character in films that suffers from the trope.

Possibly Gunn is trying to appeal to a wider audience. The same way The Russo Brothers turned a bunch of second rate heroes, into major film draws. The source material doesn't test well with non comic fans. Why this may angry you, you should still see the film. Otherwise it will fail, and wb will probably pull the plug.

I'm convinced the author is a Nolan fanboy. Nothing about the Dark Knight Trilogy is amazing. It's the same formula Nolan used in every single movie he's ever made. He makes a pretty turd, but his movies are horrible at believability and consistence.