It’s been a Liberal demand for quite some time. The drumbeat started slowly maybe a year after 9/11 and picked up a lot of steam after 2006. It was wrapped up with a lot of anti-Bush rhetoric.
Strangely, after Obama got elected, the Left has been pretty quiet on Gitmo. Or....maybe not so strangely at that. The only one really pushing the issue now is Obama.
Was there ever a purpose given for the idea that made any sense ...something to debate or argue against?