Tim Griffin of the San Antonio Express-News looked into why that might be, and it turns out it has a lot to do with cable TV:
The city also was one of the first major cities to be wired for cable in the 1970s. That gave wide exposure to the Cubs when an extensive previous schedule enabled almost every game to be shown on cable superstation WGN. The team has cut back on the number of games shown by WGN, but long-time San Antonians have developed an appreciation of the franchise for many years.
San Antonio also has a rich tradition of minor league ball, including a stint as a Cubs affiliate.
Check out Tim's full story on the Express-News website.