I don't think St. Louis is the only city where they love baseball more than football. I think San Francisco, New York, Boston, and Detroit, at the very least, all qualify for that. I hear far more about the Giants than the 49ers from online fans, and I have a lot of experience with visiting Boston, New York, and Detroit, and in all three cities I hear and see far more about baseball than football. Especially New York, as much as it pains me to say it. I see more people wearing Mets stuff in the offseason, never mind Yankees stuff, than I do the Giants or Jets (honestly, who admits to being a Jets fan anyways?).
Besides, don't you think that maybe the baseball fandom in St. Louis might have a bit to do with the fact that the Cardinals have a history of success both long-term and recent, and the Rams are...well, you know, the Rams?