Oppressing people
while partially true, there is a very big caveat, at first it wad not like that at all, it was a textbook extracvist colony, that could only trade with their colonizers directly, all foreign trade was mediated by portugal
and even after the crown of portugal moved to brazil and brazil gained status of kingdom, and in a personal union with with portugal (thus estabilishing the united kingdom of portugal and brazil), the majority of the population was still slaves and that will still be case for a long time, the now Kingdom of Brazil still operated mostly as a colony, just now the capital to which it responds to is Rio de Janeiro and not Lisbon
TBF Brazil is only richer than Portugal in terms of GDP, because it has like 15 times the population. Portugal still has twice the GDP (PPP) per capita of Brazil. Of course, in terms of politically power, Brazil dwarfs Portugal, which is probably more important for this particular issue.
It’s because Portugal treated their lands as extensions of Portugal rather than colonies. The royal family has even moved to Brasil at one point.