Because the opponents of Gitmo were using it as a political rallying point but were never invested enough to risk their own political capital in developing an alternative. It was a convenient club to swing at Bush/Cheney back in the days that Democrats were anti-war. The secrecy around the prisoners made innuendo and exaggeration inevitable, all of which could be laid at the White Houses door to rob the Bush regime of moral authority. As part of the terrorist-prison system it was a way to milk Abu Ghraib sentiment without providing any new material. And it was a way to villainize the war & security apparatus in general while that apparatus was still identified with the Republicans. Once the Democrats took Congress in 06 it stopped being a useful political ploy. The GOP was already tarred with the warmonger and torturer brushes, Cheney and Rumsfeld had lost much of their influence and Bush was already disgraced. The Democrats moved to a conservative holding strategy, riding out the next two years until the (in the bag) presidential race. Once they had the White House and both houses of Congress they werent going to rock the boat over what amounts to a foreign policy issue (not a big winner at the polls) — the administration handed out blanket immunity to the Bush era terror-warriors and put all the civil rights concerns to bed. Issues like Gitmo were no longer politically profitable. The electorates first concern is generally their personal economy, and doubly so after the 08 crash. When it came to relocating Gitmo prisoners all the Democrats could see were potential vulnerabilities: What if a prisoner escapes from their domestic prison? What if a freed prisoner becomes involved in a terror group? The politicians who were safe in a crowd protesting the Bush regimes policies now realized they could be held accountable for their individual votes on any particular shut down plan. Political support died, Congress passed some nonsense blocking the Commander In Chief from managing a military prison, and the world kept turning as most Americans paid more attention to their wallets than they did to any nonsense about human rights.