In Derrick Goold's article explains that the rivalry between Houston and St. Louis didn't have anything to do with not trading Roy to the Cardinals.
"That was not a roadblock. I think (trading within the division) used to be something you avoided. That's not how I viewed it this time. We had discussions with the Cardinals, and it just did not work - it did not work for either team. It just never really materialized. They talked two or three times, and it never got close to being a deal."
McLane acknowledged that Oswalt was "excited about the potential of being with the Cardinals" and that he expressed that wish to Houston ownership. McLane said the players offered by the Cardinals, who included at least one major-league player, did not cause negotiations to falter.
Hmm. So it wasn't the rivalry, and it wasn't the players. What was it, exactly?