Smooth Jazz format is huge with minority women 25 plus, and is known as a "passive" listening experience in offices, and work stations. St. Louis is only 18 percent black, so having such a small percentage relative to other big markets, add in the women in that category who are listening to other minority oriented radio stations, i.e. hip hop, or gospel, and the base is not much. Also, much of smooth jazz is very east or west coast artist based, namely New York-Philly, and Los Angeles.
St. Louis is a large, but very provincial midwestern city with a strong swing towards country and rock music. Lots of emphasis on sports and talk leaves a very small potential audience to reach out for today. Perhaps a small AM could make it work, despite the disparity between ultra high quality recording techniques used for smooth jazz, and the usual inferior sound quality the AM band is known for.
I think it's a simple matter of research which shows what I've pointed out here, and a corporations unwillingness to devote a big FM signal to such a limited chance of success. After all, we had one until fairly recently that was excellent, and won national recognition, but it got blown up unceremoniously anyway.