I think it's because Putin helped Iraq do it, the revelation could topple him, and Bush is more worried about what kind of nationalist/fascist government might follow him and gain control of the Russian nuclear arsenal than he is about revealing the truth - no matter how advantageous the truth might be to him, politically.
Moreover, I think Democrats on the Senate Intelligence Committee are well aware of this, too. Remember a couple of years back when some of them were cautioning their more Leftist colleagues not to push too hard on the "Bush Lied - No WMD" angle (it's been primarily the MoveOn activist types and the MSM echoing that line). It seems to suit everyone to shut up about it, for now.
A reasonable guess. That's also the opinion of General Thomas McInerney. (He's expressed this view several times on FNC).