It's all about the money. More specifically, it's about the wealthiest fans' money.
In February, the NFL commissioned a study (from Conventions, Sports & Leisure International) to gauge some important financial aspects of the St. Louis market and the region as a whole as it pertains to the ongoing effort from Governor Jay Nixon's task force to finalize a plan to build an NFL stadium on the St. Louis riverfront.
The St. Louis Business Journal obtained a copy of the EXSUM (pdf available here), used for discussion of the study itself, which offers some legitimate concerns about the market for fans hoping to retain the Rams in St.