After you mentioned this I went to verify what you found and I found the actual SOFA agreement signed 17th November 2008. You can read the actual SOFA Here.
http://www.state.gov/documents/organization/122074.pdf
I kept on looking why Obama did not fight harder to get a SOFA signed to have troops remain in Iraq and all I found out was that the Iraqi government leader-ed by Al Maliki would not grant immunity for troops stationed in Iraq.
I also found this interesting fact:
The agreement failed over a demand that American troops be given immunity from prosecution by Iraqis, a very touchy political issue within the Iraqi Parliament. Someexperts said Iraqi leaders may not have been willing to take great political risk with their citizens in exchange for a relatively small American force.
But no immunity meant no sizable residual troop presence.
"When the Americans asked for immunity, the Iraqi side answered that it was not possible," al-Maliki said in an October 2011 news conference. "The discussions over the number of trainers and the place of training stopped. Now that the issue of immunity was decided and that no immunity to be given, the withdrawal has started."
http://www.politifact.com/punditfact/statements/2014/aug/24/martha-raddatz/obama-wanted-keep-10000-troops-iraq-abcs-raddatz-c/