There are huge derby matches all over South America. The history of the continent's football is based on city rivalries. Unlike, say, Spain and most of Italy, the big rivalries are not between different regions of the country, but between different teams from the same city.
So why does Boca Juniors vs. River Plate stand out?
There are two explanations. One is a consequence of the importance of Argentina to Latin American football. The game was introduced, especially in the south cone, by the British. But it was Argentines and Uruguayans who did much to carry the sport north.