His reason? Would he say something like, 'Cuz I screwed up'? or, 'I was unable to effectively do my job'? Nope. He said;
"(My resignation is) ....in the best interests of the President".
Couldn't have put it better myself.
So first he gets 'reassigned' because nobody ever, ever, gets fired (or held accountable) by Bush for screwing up, and it looked like it might be a tougher sell to give him the medal of freedom than it was for Tenet et al., so having him resign to take heat off of Bush is at the least a refreshingly honest take on why he had to go.