Like others on this thread have said, this isn't really a shocker.
For one, I agree with all of those who've said that Bill seemed to genuinely like him. And from a coach's perspective, especially in the 2 offseasons that Parcells had him, what wasn't there to like? Carter practically lived at Valley Ranch studying film and working out, dieted down when Bill asked him to and actually lost extra pounds beyond what Parcells had requested, and generally seemed to want to will himself into a leader on the team.
Second, because of all the offseason work Carter had put in during the winter and spring of 2004, I really think Parcells believed he was ready for a breakout season. Even though that may be readily ridiculed by many on this board, an objective look would tell you that Carter seemed to have sufficient physical skills if his mind was right to improve on his 3300 yard season of 2003 during that 2004 campaign.
Regarding the comment that it's hard to believe Jerry burned a 2nd round pick on this "clown", Carter produced better results than Joey Harrington or David Carr have over a longer sample size of playing time so far in their respective careers. While it's unfortunate the 2nd round pick ended up a waste due to whatever personal issues Carter had, think of how the Texans to an extent (although they're still hanging on to Carr) and the Lions might feel about the spots in which they took those guys.
Incidentally, I've never been a Carter lover or hater or anything else, I am simply a Cowboys fan who roots for each and every player that dons the uniform of the team. I never understood the emotional attachment on either end of the spectrum to the Carter issue, I'm just trying to give what I think is an objective opinion on why Parcells may have felt it was so hard to hand him a pink slip.