When two baseball teams share a city, one of them dominates the geographic region with the city as its center. Greater New York, upper Jersey, lower CT like the Yankees, not the Mets. Northern California likes the Giants, not the A’s. In SoCal you won’t find many Angels fans outside Orange County itself. And the whole mid-northern Midwest, from Iowa across to central Indiana, roots for the Cubs, not the White Sox, whose fanbase consists of southern Chicago and a few adjacent suburbs.
(Go here for an amazing, data-rich, zoomable interactive of this NYT UpShot map, but be prepared to be depressed about how many Yankee fans there are freaking everywhere.)
Why? For NYC, LA, SF it’s pretty clear; one team is older and has a historic base that the other lacks. But for Chicago it’s less clear.
One friend suggested that Iowa has a, um, relevant ethnic similarity with the part of Chicago containing Wrigley Field. But Chicagoan tell me that the ethnic identification of White Sox fandom is historically Irish, not African-American.
My best guess is that it’s WGN, a mainstay of basic cable for decades which may have spread Cubs fandom across the nation the way TBS did for the Braves. But then one asks: in 1950, before TV, was there more parity between Cubs and White Sox fans? Who did people in Des Moines and Indianapolis (and for that matter Milwaukee and Minneapolis) mostly cheer for back then?
And what about New York, back when there were three native teams of about the same age? Did fans in Poughkeepsie and Rahway split evenly between Yankees, Dodgers, and Giants? What about the Phillies and the original A’s?
Quomodocumque 
Actually, what I said isn’t quite right — in 1930, the Giants and the Dodgers were significantly older than the junior-circuit Yankees. So I wonder whether they had a broader geographic distribution of fans!
Although, in 1930, while the Yankees were a couple decades younger than the Giants and Dodgers, they were also arguably the more exciting team. Certainly they had superstar Babe Ruth while nobody else did.
Looking at the map closely, it really doesn’t appear that Cubs fandom has a national spread: cubs appear as one of the top three teams only in the area centered around Chicago, which is only slight larger than the colored region where they are #1.
The real mystery is all these Yankee and Red Sox fans: apparently, both teams have a baseline support of 5-10% almost everywhere in the country (their support is weaker only in the strongholds of the some of the older teams). Very oddly, both teams have strong support in the rural south and mountain west. How does that happen?
One theory: besides ethnic identities of the population, there’s simply a lot more money in North Chicago than South. Therefore, maybe Cubs could charge more for tickets, which affords them better and more publicity, etc.
Before TV, I reckon people in Iowa didn’t pay all that much attention to Chicago teams, preferring instead local teams in the smaller leagues.
Before TV there was radio! Major league teams had regional (or perhaps broader) “networks” of local radio stations which broadcast all their games. Part of my childhood was spent in a rural part of Missouri, not that close to St. Louis, but where Cardinals fans dominated. Farmers could listen to a game while working. This was also the era of the great baseball announcers (ever heard of Dizzy Dean?). Even in somewhat isolated areas there was awareness of the major league teams. A highlight of one of my childhood summers was getting a Stan Musial baseball card. I think I must have chewed a lot of bubble gum.
There is a lot of flow of young people (especially college grads) from other areas of the midwest into the north side of Chicago, and many of them leave eventually to move to other parts of the midwest. In general the Cubs fanbase is more mobile and more connected to neighboring regions than the White Sox fanbase, who are more likely to be from the area. Add WGN on top of this (which among other things allows people with some connection to Chicago to keep up with the Cubs) and the Cubs end out with a wider region of support.
Des Moines is also the home of the Cubs’ AAA team, the Iowa Cubs, and this is the closest thing to a major league team (in any sport) that Des Moines has. In my experience that explains a good chunk of central Iowa’s fondness for the Cubs.