Why do we call it "gum arabic" and not "arabic gum"?
As with many postfix adjectives in English--e.g. attorney general--we can blame French (or, at least, the Normans) for this one. The OED points to both Anglo-Norman (compare Middle French gomme arabique) and post-classical Latin (gummi Arabicum) roots. So, Dan is more or less correct that Latin is the ultimate source, but we imported the construction as early as the 13th century.