The midwest is already an incredibly underserved region for IKEA with only five stores (two outside of Chicago, two in Texas, and one in Minnesota). St. Louis is the perfect location for a new IKEA, to see why keep reading.
First of all St. Louis is a college town with many students living in apartments. Between St. Louis University, University Missouri St. Louis, Washington University, Fontbonne, and Webster (plus a few more I'm forgetting) that's a lot of students who are hard pressed to fill massive St. Louis apartments with furniture on a budget (although students who aren't on a budget don't have it much better).
Additionally, downtown St. Louis has an explosive loft market. Seemingly dozens of downtown buildings are being turned into lofts with more being added everyday. One listing shows over 50 buildings and I'm sure there are many more. All these people are going to need furniture, and while many can afford and will shop in stores like Centro, UMA, Blend, and Mossa, not all will, beyond a few prominent pieces.
St. Louis also has a growing appreciation for modern architecture and design. The Pulitzer Foundation for the Arts was designed by Tado Ando, next door is the Contemporary Art Museum an architectural achievement by it's own right, and construction is nearing completion on Fumihiko Maki's new Art and Design building at Washington Univerity. Plus there are rumour circulating of Daniel Libeskind taking on a major downtown replanning project. IKEA's modern, efficient furniture fits right in with the developing aesthetic taste of the city.
Plus when the average sofa on stlouis.craigslist.org looks like this:
we need help...
Also if IKEA really wanted to hedge their bets they would build a store in Jefferson City, which is halfway between St. Louis and Kansas City. Kansas City also has tons of students, lofts, and loves modern design.