My guess: Larger % of people living in city centers vs rural areas. A quick glance at population data over time seems to back that up, but you know what they say about correlation.

Maybe has something to do with the type of colonisation in each country. Argentina had its first university in 1613, while Brazil had its first in 1808. Brazil as a colonial project kept its extractivist nature for a lot longer.

It's a really good question, I'm from Argentina and I don't have a solid answer. Like someone else said the colonization and history is likely to be part of the reason, but I would like to remark that Argentina has a lot more influence from Italy, Spain, Germany and France, and also Jewish ( iirc Buenos Aires used to be the city with the largest jewish population after New York in the 90's). It's a very different cultural melting pot compared to any other country in South America.

I mean, Br and CL made a lot more progress. What gets to me is Mr Latam LGBT is behind Argentina by one measily point

Argentina used to be first world as well, for whatever that's worth.