Because for some reason DFU won't work unless you download something from the ADC site.
Fixing DFU mode on 10.5.6
As noted previously OS X 10.5.6 introduced a bug that affected the use of DFU mode. with some Macs. There have been previously published hacks and techniques to fix this, but here is another method that can be used to temporarily restore DFU functionality in order to use QuickPwn or PwnageTool.
You will need an account with ADC (Apple Developer Connection) this is free and takes a few minutes to sign up, you should read the terms and conditions carefully and you should only sign up if you are thinking of developing applications in the future -
http://developer.apple.com/mac/
Download the disk image “IOUSBFamily-315.4-log.dmg” for Mac OS X 10.5.5 Build 9F33” (yes, that is a “5” in 10.5.5 - this is a developer debug package of the USB kernel extension).
Unplug non-vital USB equipment, such as external DVD writers, USB scanners, USB mass storage devices, at the most leave a Keyboard and Mouse connected.
Install IOUSBFamily-315.4.1.pkg from within the disk image
Reboot your system!
Perform necessary DFU activity with QuickPwn or PwnageTool.
Download the disk image “IOUSBFamily-327.4.0-log.dmg” for Mac OS X 10.5.6 Build 9G55”
Intall IOUSBFamily-327.4.0.pkg from within the disk image
Reboot your system!
Reattach your USB peripherals.
So download the 10.5.5 image and run that and then try. I just tried restoring to 2.2 but it didn't work and I was stuck with a screen telling me to connect to iTunes. I just restored from the 3.0 ipsw instead and that worked, try doing that.