The above map shows the geographical distribution of fan support for MLB teams. I thought it was interesting when I saw it and thought I would share. A quick note: gray areas are those in which there were not enough respondents to give them a color. Some of that may be because those areas are sparsely populated. So I think it's safe to color in places like Southern Missouri and Northern Arkansas as Cardinal country, even though the areas on this map are gray. In the Mountain West (outside of the Denver region) such extrapolation may be more difficult, and in fact baseball may simply not as much reach in those places.
A few thoughts: look at the base of support for the Braves. Considering that they are a relatively new team (moved to Atlanta in 1966), the coverage is impressive. But much of it may have come at the expense of the Cardinals. Until the Braves moved to Atlanta, the Cardinals were the southern-most team in MLB. Not too long before that, they were also the western-most team. That, combined with the reach of KMOX, created many Cardinal fans in the South and West. My father, for example, grew up a Cardinals fan in Raleigh, NC because he could listen to the games every night (having Musial and Red and then Gibson and Brock on those teams could't've hurt either). Granted, the plural of "anecdote" isn't "data," but I get the impression that my father's experience was not rare.
However, as more teams moved South and West, and as expansion put teams in places like Kansas City and Houston, a large part of the Cardinals fanbase was cut out. This process largely occurred in the 1960s and 1970s, but it may have had the greatest effect on the children of the 1980s. By then, the teams that had moved in the 1960s and '70s had largely settled, and the "KMOX effect" that had been so prevalent earlier in the century gave way to the "Superstation effect" by which children in the South could watch and follow the Braves on TBS (and those in the Northern Midwest could follow the Cubs on WGN). The parents may have remained Cardinals fans (or not), but their children would be drawn to the Braves, or the Royals, or the Astros, or the Cubs. Further expansion into Florida and Colorado regionalized fan bases even further.
The Cardinals still have a very large geographical population; at a glance, it even appears to be larger than the Cubs' footprint. It has long been a point of pride for the franchise that the team unified a "Cardinal Nation" that stretched all across the United States. But the territorial reach of that Nation is nothing like it used to be. This may be to the detriment of the team, as the opportunities for marketing and merchandising have grown steadily over time, just as the regional reach of the club has shrunk. But it's certainly been good for MLB overall, since greater regional engagement with fanbases have raised the profile of the sport in the South and West, which are now some of the richest regions of the country.
Anyway, I just thought the map was interesting. I have long been interested in what I call the "Sociology of Baseball" and how it has changed and reflected national trends over time.