Many of the trim combo items were limited in customization because they were preassembled or came from somewhere else in a package. So they just didn't offer the option of a full ala carte. So maybe the wheels came to the final assembly from different places, or with a deluxe trim package, and the wheel covers were all ready accounted for? In other words they would have had a bunch of extra wheels they didn't need so they just stopped that option.
And then sometimes you could totally customize. It is certainly odd. Other times I think they just didn't anticipate that need/want.
