Some people don't care about the iPhone 4. The 8GB 3Gs still is an excellent phone and people do want it.
I got an iPhone 4 on launch. A few weeks later. I sold my original 3GS on one of my lines and took the money and bought a new 8GB 3GS and a new 32GB 3GS. The 8GB was shipping in 7 days back then in July so I ordered from AT&T instead and had it the very next day.