According to Sony's product planning division head Naoya Matsui, "We'd planned to release a PSP model without a UMD drive since the very beginning."
Further elaborating on this statement, "But if we'd simply release the hardware, there wouldn't have been much for everyone to enjoy," he said in an interview with Japanese site GameBusiness (linked above).
To quote: "We needed to prepare the right environment for it first - things like the transferral of content with the PS3 and PSN, and PC software to manage content like music and movies such as Media Go."
Of course, they had to wait first until digital content is at a competitive playing field with physical media, and now the time is right.
The PSP Go will simultaneously launch in the US and Europe on October 1, while Japan will get it a month later.