In a succinct, informative Macworld article, Glenn Fleishman explains why.
Simply put: for pulling down data, the 3GS can use AT&T's newer HSDPA service, at 7.2 Mbps. The corresponding upstream service is HSUPA, at either 1.4 or 1.9 Mbps. AT&T is building out this higher speed network.
But the 3GS can't use the faster upstream service. And why? Fleishman notes the smartphone has a UMTS/HDSPA chip for the cellular connection, the earliest 3G implementation on GSM networks. Maximum rate: 384 kbps.
He includes a link to TesMyiPHone so you can check it out yourself.
I liked his closing: "But I suppose this offers a new marketing bullet point for a future iPhone model—now, with HSUPA!"
Cox is a senior editor at Network World.