I believe that the problem is that Al Franken wasn't sworn in until well after that session was well under way, Senator Ted Kennedy was missing for many votes due to his brain cancer, and Arlen Specter didn't switch sides until much much later. There were a few other Democratic Senators who were either out or "Blue Dog" and "DINOs" - the Democratic "Party" is actually more of a loose coalition. The Democrats had the seats, perhaps, but nothing more, for a total of 72 days.
(Reprinted from the last time I did this comment.)
The problem in closing Gitmo is that there have NEVER been enough people in Congress who are willing to take the political hit of letting anyone leave; witness the fact that we captured Chinese Uyghurs back in 2002, determined they weren't terrorists in 2008, and FINALLY released the last of them in 2014. These were GUYS WE KNEW WERE INNOCENT FOR SIX YEARS and still hadn't let go.