You're forgetting that Obama tried to stay in Iraq - it was the Iraqis who wouldn't agree to immunity from local prosecutions that undid the deal. I doubt Bush or anyone could've achieved otherwise while maintaining the pretense of a non-colonial enterprise.
A messy Iraq, politically dominated by Iranian clients, was nearly a foregone conclusion before the war started.
De Selby beat me to it. Bush and Obama were all for leaving combat troops in Iraq if the Iraqi government (Maliki, the PM) had signed a SOFA. Talabani, the President and a Kurd, tried to push them to sign it but since the power is with the PM it didn't get done.
We've been over this before. Political insiders from both the Obama admin and Maliki's side have stated that a SOFA deal was possible. Al Maliki himself is on record saying that he was willing to deal, all he needed was some concession from Obama to save face. Obama wouldn't negotiate, not seriously, because he had already decided he wanted a full military withdrawal for political reasons.
The SOFA was the pretext, the excuse, for the US withdrawal from Iraq. It was not the reason for our withdrawal.
In any event, that just covers our military abandonment in Iraq. It does not explain our simultaneous diplomatic and political abandonment. No excuse has yet been offered for that fuckupery.
It's all part the O Admin's gross incompetence in foreign policy. Iran, Israel, red lines in Syria, no-showing in Paris, busts of Winston Churchill, you name it. The Obama Admin has a long standing history of royally screwing up foreign relations, from the simplest stuff to the most important. The lack of follow through in Iraq is just more of the same.