my new favorite theory about why no one other than Obi-Wan ever cites the Yavin Code, aka the GFFA's version of the Geneva Convention, is that the only copy is an ancient manuscript housed in the Jedi Temple, to which access is tightly restricted
most Jedi have never had any reason to peruse the document, due to the combined factors of how poorly recognized the Yavin Code is and how rarely a Jedi is likely to need to know conventions for the treatment of prisoners of war, etc
but Obi-Wan, with his history of involvement in large-scale conflicts since the very beginning of his Padawanship, had ample reason to want to know what it says
(un)fortunately, Obi-Wan is also acutely aware of how poorly understood the Code is, and is a good enough Negotiator to take Full Advantage of the opportunities for his trademark Half-Truths and Hyperbole to fill in those gaps in other's knowledge