I'm struggling to put my finger on why they didn't deliberately pitch it to the SF/F crowd. There's plenty of us out here. I heard that some PR dude or other actually changed the title, taking out the "Of Mars" that was originally attached to it, in order to give it a broader appeal to people who aren't into SF/F. I find that presumptuous at best, and insulting at worst.
(I've also been inspired to do a blog post on why genre movies seem to get so little respect, just for being genre movies.)
Simply, it cost too much. If it had been a $30 million film, they could have marketed it as a full-on sci-fi and made their money back. If it costs $250 million with another $100 million for marketing, it has to be sold to the broadest audience possible. The four quadrants as they call it. That's absolutely non-negotiable for them.
And, like you say, that's also why they dropped the 'of Mars' because, rightly or wrongly, they believe that there is a group of people out there who will be turned off by any flavour of sci-fi and they cannot afford to turn anyone off from seeing this. (Hell, it didn't even
look like Mars from the trailers, it looked like the same, bland generic desert from Star Wars, The Mummy, Prince of Persia.)
The problem is that all this left them with a title that didn't really mean anything to the majority of the people they were trying to entice. I'd never heard of John Carter before this film, and I didn't care once I'd heard it. That's not to decry the books or the characters, I'm sure it's wonderful stuff, but it does appear to be the overall reality of what played out in the end. For this level of budget, your audience has to either be very aware of the material (Titanic, Star Wars) or so enticed by what they're seeing (Avatar, Inception), mixed with a healthy dose of positive hype/controversy, (again, Avatar:
the most exepensive film ever made!!1! ) that they are ultimately drawn to going to see it.