At some point, the project became a live-action movie and Paramount ended up with the rights in 2002. Several directors were attached to it at various times under Paramount's care: Robert Rodriguez (Spy Kids, Sin City), Kerry Conrad (Sky Captain and the World of Tomorrow), and Jon Favreau (Zathura, Iron Man), for example. Apparently, Paramount let the property go last year and it was homeless until last month when Pixar started showing interest.
Pixar's involvement re-opens the question about whether or not Disney plans to turn John Carter into a live-action franchise as Edgar Rice Burroughs' estate says (and Variety reported), or an animated feature. Disney's keeping quiet about it for now.