From wmpoweruser.com, find below 3 speculations on why HTC may have decided to do this:
1) AT&T demanded that the HD stay out of USA, to prevent any competition with the iPhone 3G. In return they would carry the much delayed AT&T Fuze. This is supported by the absense of the HTC Touch Diamond on AT&T, indicating they do not welcome good competition to their favourite phone.
2) HTC did not want to draw attention from their new G1 android phone, and therefore decided not to release in USA.
3) HTC thinks the investment would be wasted, due to the local dominance of the iPhone in USA, unlike in Europe, where the seemed to have accelerated its release.
I'm going with reason #1. Apple and AT&T are the devil and the devil's spawn, respectively.
Looks like XDA-Developers is going to have to start calling on some HARDWARE EXPERTS!! Swap out the radios perhaps???
Very disappointing news, but hopefully this just means that we'll get something even better when the Touch HD II is born!