Why is Brazil's soccer team so successful?

In a Word, "Pele".

It is truly amazing what the inspirational accomplishment of one man can do for those for whom his accomplishment carries meaning and some identification. It is probably worth investigating what brilliance Brazil showed in other sports or arts, etc., before the advent of Pele just prior to 1960. That is, what personal achievement did the Brazilians identify with and just how good was their soccer, in particular, in [say] the 1930's and 1940's and early 50's?

When Pele showed his wizardry on the soccer field, other nations with money wanted to buy him for their professional leagues. Fortunately for the Brazilians, their president declared Pele "a national treasure", and he was unable to leave Brazil. He took soccer to a new level.