St. Louis has always had a much bigger market than Cincinnati. It's because there really were no other MLB teams west of the Mississippi for so long, and because of the strength of KMOX, which reaches dozens of states. There are Cardinal fans all over the Midwest, in states like Kansas, Wyoming, Idaho, Iowa, Montana, etc. The Cardinals were the Yankees of the NL for quite awhile. The Cardinal nation is huge.
According to this site, Cards are 21st biggest media market and Cincinnati is 35th.
And the Saint Louis metroplolitan arena is much bigger than Cincinnati's, it's around 1 million people bigger, around 3M to 2M.
Their market is bigger, their fan base is bigger and the number of people driving distance of them is bigger.