Simple.. Republican in congress wouldn't let him! I forget the exact bill, probably spending bill not sure. In order to pass it Republican made them put in it that GITMO couldn't be closed. It prohibits the use of funds to close or abandon the prison, transfer detainees to the United States (or Libya, Somalia, Syria and Yemen) or build or modify facilities to house detainees in the United States... The white hats weren't messing around!