Just my opinion, of course, but bin Laden escaped because we never really had the full cooperation of Pakistan.
I'm sure Dubya threatened Musharraf on 9/11; we've read what I think are credible articles to that effect. That's why the guy cooperated with us to the extent he did. It was unprecedented. Unfortunately, the tone of those threats must've dropped off somewhat, and thus the heat on bin Laden lessened and he was able to find lasting sanctuary in western Pakistan.
We would've had him by now if we had pushed Mushy a little harder.
Note for the Bushies here -- of whom I am one -- I did not say Bush made a mistake! Bush had good reason to reward Musharraf for his cooperation. I just wish he hadn't seen it the way he saw it... At least, that is, until we had bin Laden's head on ice.
And why don't we have Zarqawi in Iraq then? - Who haven't we pushed hard enough to get him?
Nonsense. (IMO)
The Pakistan border region is basically autonomous from the rest of Pakistan (has been for over a century) - Additionally people just don't have a clue as to the terrain in this region of the World to suggest we could simply catch him (UBL) if only the Pak's put more soldiers there. The terrain just doesn't allow for Operations like that, nor would those troops lead to any guarantee of capture (at all) - Nor would the Pakistan public stand for any long term, large, occupying Pakistan military operation within this region to boot.
Again, we control 90% of Iraq, we have over 125,000 soldiers there, we have 85% support from the new Gov't and yet we still are hunting down Zarqawi. These Ops are much more involved and fluid then 90% of those discussing them understand.