I've answered this here. The gist of it is that the Portuguese royal family and court moved to Brazil to escape Napoleon in 1808 while Brazil was still a colony, and this allowed Brazil to stay united under a single monarch when it became independent in 1822, while also creating a strong central government that could squash separatist attempts.

And in regards to the Spanish territories, when they were broken down into nations, usually out of large colonies like New Spain, they were mapped around Hacienda's, Estancia's, and to some extent the Church's property. This is why there is some debate as to weather or not Latin America has truly had a chance to form and undergo state-building like Europe, where small enclaves like the Duchy of Westphalia ceased to exist when taken over by a more stable state (the Prussians). If you're curious as to the size of an Hacienda in Spanish Latin America, one notable hacienda stretched from Central Mexico into the modern US. These were not typical but did exist since land grants and permission to govern an area required lots and lots of money paid to the king.

Politics in Latin America: The Power Game, 6th Edition (2017). Gary Prevost and Harry Vanden. Oxford University Press.

I gave a brief summery of Benedict Andersons theory here.

Basically the Spanish colonies were too far apart to have a shared national consciousness.