I believe the difference you are likely referring to is related to the Dock-N-Lock connector on the back of the radio. Most of the time, these modules are connected between the radio and the factory wiring harness. This simply isn't easy to do (although not impossible) with DNL connectors. Many of the GM radios share the same connectors on the back of the head units which makes it easy for aftermarket manufacturers to integrate their products across several platforms.
The Bonneville is one of very few GM vehicles that use the Dock-N-Lock connector. This is also shared by some models years of the Oldsmobile Aurora and the Buick LeSabre. I'm not sure that any vehicle after 2005 was produced using this connection system. Therefore, it doesn't make a lot of sense for companies to produce a product for the older, out of date system.
Based on my own work with OEM head units, I suspect that in many cases these modules will work, but there would need to be some significant wiring changes in the vehicle for it to happen. In the newer vehicles, you are also dealing with the high and low speed GMLAN bus as opposed to the Class 2 Serial Data Bus. This changes things quite a bit as well.
StingRay Bike w/cards in the spokes, Weeeeeee!