There are numerous military bases in the continental United States that are on the chopping block by Congress to be closed. Some of them are in the area of Democrats and if closed could seriously affect their status in Congress.
Since GITMO is not in the continental United States or a terratory it cannot affect the outcome of a some of the members of Congress due to the possible loss of votes from constituents.
What they fail to realize is the strategic importance of Guantanamo 0ver the political importance and economic importance.
I have even heard a rumor to the effect that if Guantanamo is closed there can be a considerable amount of money made by transforming the military base into a resort not taking into account the military importance it has to the United States of America.