Because it made it easier for the Iraqi parliament to approve at the time. Yes, it was kicking the can down the road a bit but sometimes you do that.
Google it and you’ll see multiple sources saying it was expected to be renegotiated. I don’t think this is especially controversial.
http://www.nationalreview.com/corner/380508/no-us-troops-didnt-have-leave-iraq-patrick-brennan
The idea that the SOFA was supposed to be renegotiated after it was signed is ludicrous on its face. If the SOFA was drafted a certain way in late 2008 as a matter of "political expediency" for the benefit of the Iraqi government, then what exactly was supposed to happen between 2008 and 2011 that would make it any more politically expedient for the Iraqi government to allow U.S. troops to stay there?
The only political expediency in that agreement was for the Bush administration and for the McCain presidential campaign. The U.S. mission in Iraq was an financial and political disaster for the Bush administration by the time 2008 rolled around, but nobody could publicly admit that without jeopardizing McCain's chances to win the 2008 election. That's why the SOFA was signed in November in 2008 when neither Bush nor McCain would ever have to answer for it.