Tarzan had the benefit of being easy to translate to film and other mediums. He's just a guy in the jungle. John Carter takes a lot of effects to render the supporting cast alone, much less the vehicles and flora and fauna of Mars.
According to wiki, there were 89 movies with Tarzan in the title between 1918 and 2008. It also doesn't hurt that the idea of the feral child with abilities beyond the normal person is an old and common mythology, so it crosses cultural in generational boundaries better than a guy with the initials JC mystically traveling to a scientifically impossible version of Mars to be their savior.
I prefer John Carter as well, but I can see why it's not a draw for the average person.