This might be slightly over-simplified, but basically this is because of contractual "tie-in's" between the major conferences and the bowls (assuming, of course, that TCU is otherwise deserving of a "better" bowl, which is arguable but supportable).
These tie-in's involve the better, non-BCS bowls (such as the Holiday, Cotton, Peach, etc.) having mutual deals with the major conferences, so that any given bowl knows they will never have worse than, for example, the 3rd best team from the Big XII vs. the 4th best team from the SEC. The result is that you will never see a team like TCU (from a "non-BCS" conference) ever go to a particularly good bowl unless they qualify for an actual BCS bowl (like Utah last year), most likely pursuant to whatever objective, "automatic qualification" criteria is currently in place for that.