Because if you're in Argentina, you can only sell them for Argentine pesos, which aren't a clear winner over bitcoins.
Sure, BTC might win out of Argentinan Pesos, but it's certainly harder for the average merchant in Buenos Aires to accept and transact using BTC vs. paper USD. The barrier to entry to accepting foreign USD for transactions is 0; for BTC the entry is more difficult (currently).
If Bitcoin is legal, easy, and cheap to use in developed economies, it doesn't matter where it starts being the most useful. It will spread everywhere.