could this be the reason?
http://www.globalsecurity.org/military/facility/iraq-intro.htm Iraq Facilities
A 20 April 2003 report in The New York Times asserted that "the U.S. is planning a long-term military relationship with the emerging government of Iraq, one that would grant the Pentagon access to military bases and project American influence into the heart of the unsettled region." The report, citing anonymous sources, referred to one base at Baghdad's international airport, another near Al-Nasiriyah in the south
, the third at the H-1 airstrip in the western desert, and the fourth at Bashur AB in the north.
There had been several statements at that time about the possible duration of the US military presence in Iraq. Mr. Richard Perle mentioned six months; Ahmad Chalabi, two years.
American officials have tried to make the point that the US presence in Iraq will not be a permanent or long-term one. US Defense Secretary Donald Rumsfeld in a 21 April 2003 press conference said that any suggestion that the United States is planning a permanent military presence in Iraq is "inaccurate and unfortunate." Rumsfeld said "I have never, that I can recall, heard the subject of a permanent base in Iraq discussed in any meeting. ... The likelihood of it seems to me to be so low that it does not surprise me that it's never been discussed in my presence, to my knowledge. Why do I say it's low? Well, we've got all kinds of options and opportunities in that part of the world to locate forces, it's not like we need a new place. We have plenty of friends and plenty of ability to work with them and have locations for things that help to contribute to stability in the region. ... Rumsfeld: I think there is a down side. I think any impression that is left, which that article left, that the United States plans some sort of a permanent presence in that country, I think is a signal to the people of that country that's inaccurate and unfortunate, because we don't plan to function as an occupier, we don't plan to prescribe to any new government how we ought to be arranged in their country."
On 23 March 2004 it was reported that "U.S. engineers are focusing on constructing 14 "enduring bases," long-term encampments for the thousands of American troops expected to serve in Iraq for at least two years.... The number of U.S. military personnel in Iraq, between 105,000 and 110,000, is expected to remain unchanged through 2006.. the US plans to operate from former Iraqi bases in Baghdad, Mosul, Taji, Balad, Kirkuk and in areas near Nasiriyah, near Tikrit, near Fallujah and between Irbil and Kirkuk... enhance airfields in Baghdad and Mosul..."
more.....
Seems they are going to need troops and personnel on all those bases.