^^ like i said it just has to do with colonialism
england n france got north america, hence the US and Canad speaking english except for pockets of frenchs spoken in places like quebec n parts of louisiana
and spain got Central and South America sans places like Brazil where it was controlled by the Portuguese and French guiana which was obviously under french control
from what i gather since the languages are so similar most brazilians can understand spanish but have a difficult time communicating in spanish if they arent otherwise taught some spanish but its similar to us in the US or more specifically places like texas where a percentage of the population has learned spanish as it increases business with neighbooring countries.
but i think the reason why spanish didnt really spread beyond the boarders of colonialism isnt because of mountains or the terrain of the forrest but b/c the language of the colonizers was used msotly formally when dealing with the state , like it was eventually learned n adopted by most within a certain colonial area but all of south america was ameriindians before the europeans came, so say some indigenous amazonians in peru might have learned spanish (most probably have by this day n time) but still in day to day life they spoke/speak their native language just the way their neighboors in brazil spoke/speak their native tongue while also knowing how to speak portguguese for official things regarding the state