It was kind of a boneheaded idea. All the evidence apparently was that people "think they hate sci-fi" and anything with Mars in the title is the kiss of death. But by changing it to JOHN CARTER (along with Stanton's stupid explanation of how he changes it at the end and *EARNS* the title) audiences were just confused. This is NOT Harry Potter and it not merely enough for Andrew Stanton to love these books as much as kids and younger people today love Harry Potter. The material definitely had potential but Stanton didn't make it accessible like say Avatar and Star Wars were accessible. Stanton also thought putting a lot of characters and ideas that didn't come to the forefront in later books (since apparently as I understand it Burroughs hadn't even really thought of them yet when he first wrote Princess of Mars blah blah blah) to try and make the plot bigger. Instead it only confused the audiences even more.
This needed to be a much smaller and simpler story for about half the budget. It needed to slowly bring you into the world of John Carter and keep all the bigger ideas for later. It basically needed to be the first act of a larger story.
Just take the prologue for example. Stanton probably knows all these characters and conflicts back to front, but a general audience or kid has never read this stuff and has no idea what is going on. Parents are probably scratching their heads at the start and have no idea what is going on. I had no idea what is going on. Dominic West's character sort of looked heroic and like the good guy at the start but apparently he's a bad guy. Then these weird immortal dudes that can control everything give him a weird thing I can't explain. It doesn't make any sense. And the Therns in the movie also make no sense. The Therns should've been completely cut from the entire film. Maybe they are way in the background or referenced or briefly seen at certain points but to make them such active parts of the plot literally killed a lot of the movie. No one understood the Therns, their powers, or the Ninth Ray nonsense. It was way too much to throw at people and expect them to understand.