With the exception of Canada, all these countries end in 'ia'. Arriving at their 'ian' adjectives involves the addition of one extra letter: 'n', which does require adding an extra syllable to the word. Argentina does not end in 'ia', it ends in 'ina', and trying to maul it into the adjective Argentinian demands the addition of an unwieldy extra syllable. You wouldn't refer to people from Botswana as Botswanian. Argentinian sounds almost as awkward. It's not wrong, I often use it myself, but there is no reason to see the word Argentine as a slur and it is generally favoured in style guides as the neatest of the three options. Using the article "an" before the word "historic", however, is a crime against grammar and logic that I find physically painful!
