Simple. New ones don't justify the additional cost. I looked into it, many times. To get a similar plan for the iPhone4, it would cost me ~ $90 per month after taxes. That is $1080/year.
Now my original iPhone, which does everything I want it to (weather, maps, camera, photos, email, facebook, WRC app, ebay, craigslist, shazam, etc), but only costs ~$60 month to use. $720/year.
That's an extra $360 in my pocket, per year. That can buy a set of nice race tires, decent tool chest, etc. And it adds up. If I had upgraded to a 3GS in 2009 I'd have paid an extra $720 just to have slightly faster speeds, a higher res screen, and a better camera. No offense to newer users, but I haven't been impressed with the speeds it takes to download things, it still seems slow so I don't care if I wait a few more seconds if it already takes a few to begin with.