The next logical place to go was and still is after OBL et al.
that requires an invasion of Pakistan - the Paks have nukes, Musharaff is weak, it could spark a takeover of the country, a war with India, the nukes falling into the wrong hands, etc, etc.
Given the level of Pakistani cooperation so far, I can't see how any of these possible outcomes could be an improvement over what we have now. Sure, we might have gotten OBL, but what would we have been left with in the aftermath in Pakistan? Whatever problems we have in post-Saddam Iraq, Pakistan could have been far worse had we gone in there. And Saddam would still be in iraq.