Remember that it is also the maker of the phone that can determine a phone shortage. The reason the other galaxy s phones dont sell out is that the carriers arent pushing them as their flagship like sprint did with the evo and epic. Att made it so the phone cannot compete with the iphone and the tmobile version well its tmobile and I never see their ads. Nothing wrong with tmobile but they are not a tier 1 carrier IMHO.
There is also the simple possibility that Samsung, due to the huge demand worldwide (phone has sold over a million in first couple of weeks in China an S. Korea) is charging US carriers slightly or substantially more, and not spending much on advertising here.