Its not like South America is lacking in some truly difficult terrain or a rich tradition of guerilla warfare for race, ideology, and profit, I mean just look at the Chan Santa Cruz Republic, lasted for over fifty years as an independent nation and it was led by Mayan peasant farmers with machetes and outdated guns.
I'm not doubting the willingness of South Americans to engage in guerrilla warfare, even in such warfare lasting for decades. OTL history shows that well enough. What I am doubting, as history also shows, is the ability of most such guerrilla warfare to force out a determined, well-armed occupying power without safe havens and/or foreign support.
Even the Chan Santa Cruz republic, which you mention, had tacit British recognition for most of its existence. After the British withdrew that support in 1893, Chan Santa Cruz didn't last that much longer. (Although I'll grant that it wasn't British support which formed the republic in the first place.)
Not the best comparison but to give perspective Peru is almost perfectly set up for guerrilla warfare due its geography and that is only the most well set up. Latin America is filled with hard to reach areas that bandits and rebels have notoriously made their home in.
Latin America has plenty of such out-of-the-way areas where bandits, rebels, revolutionaries and drug dealers (those sometimes being synonymous) can establish themselves. I don't doubt that for a second.
There is, however, a big gap between "bandits hiding in the hills" and "guerrillas forcing out an occupying power". For instance, in Peru itself, Shining Path launched a wide-scale insurgency against the Peruvian government in 1980, which saw them have a presence in large areas of the countryside. Shining Path is still around today, more than 30 years later, and still haven't given up completely. But the Peruvian government is still there, and Shining Path is now little more than an organised cocaine-smuggling ring.
Much the same could be said of militant groups in Colombia, who have been at it for even longer. FARC, ELN and others have been conducting guerrilla warfare in the Colombian hills since the mid-1960s, but are no closer to taking control of the country than when they started. (Of course, one reason the Colombian rebels have been able to continue for so long is drug smuggling, but even without that, they could probably have been hiding in the hills for several decades.)
Not even to mention the fact that foreign aid is a distinct possibility if the US ever gets on the bad side of any great power (course I havent read your TL so you probably explain that bit) but I do think you underestimate how well set up Latin America is for guerrilla war.
The relative lack of foreign aid is explained in the TL. But no, I don't think I underestimate Latin America's suitability for guerrilla warfare. I do think that too many people overestimate the ability of guerrillas to drive out occupying forces (or established governments) without foreign support.
It is actually possible, by the way, to list three insurgencies which have accomplished that goal. But coming up with those three names is a lot harder than most people believe.