… home of the largest population of Japanese outside Japan.
The main answer is that unlike most other places, the Brazilians let the Japanese in:
“At first, Brazilian farmers used African slave labour in the coffee plantations, but in 1850, the slave traffic was abolished in Brazil. To solve the labour shortage, the Brazilian elite decided to attract European immigrants to work in the coffee plantations. The government and farmers offered to pay European immigrants’ passage. The plan encouraged millions of Europeans, most of them Italians, to migrate to Brazil. However, once in Brazil, the immigrants received very low salaries and worked in poor conditions, similar to the conditions faced by the black slaves: long working hours and frequent ill-treatment by their bosses. Because of this, in 1902, Italy enacted Decree Prinetti, prohibiting subsidized immigration to Brazil.
The end of feudalism in Japan generated great poverty in the rural population, so many Japanese began to emigrate in search of better living conditions. In 1907, the Brazilian and the Japanese governments signed a treaty permitting Japanese migration to Brazil.
Japanese immigrants began arriving in 1908, as a result of the decrease in the Italian immigration to Brazil and a new labour shortage on the coffee plantations.
In the 1930s Japanese industrialisation had significantly boosted the population. However prospects for Japanese people to immigrate to other countries were limited. The US had banned non-white immigration, on the basis that they would not integrate into society; these laws were specifically targeting the Japanese. At the same time in Australia the White Australia Policy prevented the immigration of non-whites to Australia.”
From an absolutely fascinating page on Wikipedia here: