The 2007 Democratic Congress. They cut the funding necessitating a timeline for withdrawal. Once it was clear we were leaving the Iraqis became more difficult to negotiate with.
Many of the bases we built-up there were permanent bases such as Camp Victory, Al Asad, Tallil, etc.. They were called COBs (Continuity of Operations Base) The US spent 100s of millions on permanent infrastructure on the COBs. The bases are still there and we’re still using some (Al Asad, Irbil, etc.) albeit with shoestring budget and manning.
"It's a quagmire if you go that far and try to take over Iraq." -- Dick Cheney, U.S. Secretary of Defense, April 15, 1994
It turns out that Iraq has simply ended up exactly the way Dick Cheney said it would. If only Dick Cheney had listened to himself, we wouldn't be here having this discussion.