The Cardinals were at one time the most Southern, and at times when the A's were in Philadelphia, most western baseball team in the US. Thus, they were the closest, and since they had the wide ranging radio broadcast, they could reach a ton of people that weren't anywhere near St. Louis.
They've also won a lot too- that helps. And they've had great players over the years that were popular- Hornsby, Dizzy Dean, Musial, Lou Brock, Bob Gibson, Ozzie Smith, Mark McGwire, and now Pujols.
The Cardinals also typically play a good brand of baseball- usually they play pretty hard, have good pitching, good defense, and they have some power and can make things happen on the bases. Baseball fans in general usually like to watch that.
Of course, in my case, my Dad is from Missouri, and as everyone knows my uncle used to pitch for them. When I was little, he would give me Cardinals shirts, and etc. and when I was little, my family would take me to some games when we were visiting family there. I would be a Cardinals fan if I lived in Alaska. My Dad and my uncle are why I love baseball so much. Also- my parents would take me Dudy-Noble Field as well, and when MSU would play in Jackson, my Dad would take me to that. We would watch the Jackson Mets too when my Mom was playing bunko. If my uncle had pitched for the Expos, I would probably be a Expos fan.
St. Louis is unique in that the baseball team is so vital to the city and so many people support it. To me, that's what makes it special, and what makes the Cardinals unique. Everyone wears red, everyone gets into it, and everyone just has a good time with it. There is a reason why you hear so many baseball players talk about wanting to play there and how they enjoy playing there. They also have a good rivalry with the Cubs, which I think also adds intrigue and attention to the team as well.
Finally- St. Louis is only seven-eight hours from Jackson, Ms. That's about the same distance as Atlanta, Houston, Dallas, etc. So, it's not like it's that far off. And to me, it's actually easier to get to and drive through than those three cities.